SOA (Springs Over Axles) 101.
What you want to do is a spring over lift. Right now it might look cheaper, a little bit of welding and some perches, shocks, be all set right? wrong.
First you will need to find the degree in which you want your perches to be placed, make sure it keeps a good pinion angle throughout suspention travel. Thats probaily the most minor of issues.
Your now also have to adress your steering. You can get away with a dropped pitman arm (75-125 bucks). That will work for mild stuff, but it will have serious bump steer. If your keeping the stock steering setup you will need to have someone build you a 2 inch drop drag link. That will cure all your steering issues, however it will leave the steering in a venerable spot... under your springs. When doing a spring over it is more then wise to invest in a crossover steering setup, which relocated the tie rod to the top of the steering knuckles instead of under them, and under the springs.
With that said driveline issues will need to be adressed without a doubt. Your transfer case is fitted with what they call a slip-yolk. It lets the driveshaft slip in and out of the transfer case when your suspension compresses from offroad obstacles or from bumps on the street. If left stock, your driveshaft will mroe then likely fall right out of the tranfer case. The angler of your driveshaft will be horrible as well, and if it were to actually drive without the shaft falling out you would eat up universal joints faster then a fat kid eats cake (no offense to the big ones). You would need to invest in a Slip-Yolk Eliminator and a rear CV style driveshaft. The SYE eliminates the the slip-yolk, obviosuly. It gives you a few more inches of driveshaft to help the angle out. The CV shaft will let thedriveshaft slip on itself and lengthen adn shorted as needed, not to mention they are much stronger and can be used on much steeper angles. A SYE kit and driveshaft will most likely run you around $250 for the SYE kit and around $100+ for the driveshaft, depending on how good of a looker and deal finder you are.
With those two things said your already spending as much as you would have on a normal 4 inch lift, and those are only the things you will need.
Another thing that you will surely feel, is axle wrap. You get that because you have all this extra torque on your springs because of the springs being on top. When you hit the gas, the rear axle, if your looking at the passanger side fo your jeep, the rear axle wants to rotate clockwise. When you take off, or are trying to crawl, it will load up like that until it cannot hold anymore and then it will unload and cause a hop. On pavement if your hit it raelly good, or the same on rocks, lets jsut hoep you have a good dentist because it will knock your fillings out. There are many ways to combat this. You can make a traction bar, which will keep it front wraping, but willlimit travel a bit, and give your rig something else to hang up on. Or build your own good spring pack. If your using stock springs for your SOA (youll get maybe 5 inches and AWESOME flex and no noticable difference in ride), take another set of factory springs, cut the eyes off both ends, and put it on TOP of your spring packs top leaf. It will almost fully cure the axle wrap problem, and trust me on that one. Or, you can buy Rubicon Express' spring voer spring. There great, but there stiff as hell... and you need to pay for them, and then you might as well have bought a 4 inch skyjacker or w/e you wanted, unless your going for built.
Another minor issue you will find when you elast expect it, is your shackles flipping. When there in the front, like they are on yj's, they let the axle spring forwards. If you have a SOA and you romp on the skinny pedal in reverse, your shackles will flip up and it iwll not only drop the front of your jeep about 3 inches, but it will stress your springs like crazy, if not break them. Get a pry bar adn flip them back ASAP if that happens. But again, to combat that, you can buy a shackle reversal. those can range anywhere from 150 to 300 bucks depending on your skills, access to a welder, etc etc.
So now you have to think about something, are you building a semi-built jeep, or building a built jeep. If you want to run 31's locked max, or 33's open max, keep the stock axles. As we all know the rear dana 35 is somewhat of a junker axle and have been scince the early days. I know everyones freidns brothers cousin ran 35's with a detriot on a d35 for 3 years and it never broke, and i know that everyones siters boyfriends uncle ran one with a 4 popper yj and 30's open and broke a shaft. The rear truth is, anything over a 33, the D35 is useless. So you can either swap in an axle with this spring over, like the ford 8.8, or use the axle and face the facts of killing it sometime soon.
The front axle, the dana 30 is a great little axle for what it is. Many people run them on somewhat built rigs without a problem. The key is to have the shafts from a 95+ jeep, becuase they use the stronger 297X u-joints (same as a dana 44) the 87-94 yj's in particular used 260X's, which are not bad... jsut much less strong the the 297's, easy swap if you find the shafts. The axle itself will hold up to a 35 without much trouble actually. The only stupid thing about them as that vacuum disconnect thing. If they work, consider yourself lucky becuase the first time it gets cold nad you need 4wd in snow, they will crap out. as mine did... 2 times.... before i smartened up and bought a posi-lock. But if your going the route of welding on perches anyways. get a dana 30 out of a cherokee, its a high-pinion like the yj's, but without the disconnect, and chances are in your favor of a 94+ having the better u-joints. Just grind off the old perches and weld on your new ones. your old steering, driveshaft, everything bolts right up, same exact axle jsut no disco.
So with alll that said and done, a proper SOA will cost you 3 times more then jsut a normal 4 inch skyjacker/fabtech/rancho/etc etc. its true, you can do a SOA for like 100 bucks, but if you plan to use it at all... do it right... and save all ur wheelin buddys the trouble of puttin ur rig back together and towin it home.
Go with jsut a normal 4 inch/3.5 or w/e lift if you jsut want a good jeep that works, with say 33's and good capabilities.
Go SOA and do axles if you really wanna wheel it, build it, spend the time and msot important money on it.
I been in the yj game for a while, and let me tell you, the only way is the right way. Not sayin the right way will never break, but it will last ALOT longer then the wrong way/easy way/cheap way.
first jeep for me was a a yj on 33's. She was locked in the back, hp d30, bumpers, 3.5 inch fabtech, coppped fenders, no gear, and i held up with the pack of everyone. Freak accident an bout 10k incl. initial cost was out the pooper. But now being around, watching friends, building rigs with friends... oh man did i learn lots...