You should be fine with 37s for most trail riding. Excellent choice going ECGS. When I did my 8.8 I had purchased an axle from a local guy on here and it was just a pain in the ass process. I got a great deal on the axle, but then I had to pay someone to weld the brackets. I did all the cleaning and prep work, painted it myself, setup all the components. I also had to locate all the extra brake parts necessary.
The only thing with ECGS that I will say is make sure to not skimp on the components. They're using junkyard axles all the same and doing only what you want them to. That said, order new brakes and calipers. Do new brake lines and e-brake cables. Also make sure
to go for new bearings and oil seals. After putting mine all together, I neglected to do the oil seals and had a bad leak.
Basically, don't cheap out because you'll just end up to do it later on at more cost.
I actually ordered all my brake components through them and I was incredibly happy with teh customer service, shipping, and the free stickers. In the end, you will spend less going this route. I'm happy all I got to learn from building and serving my own axle, but really, the build is only half the battle.
To answer your other questions, you should definitely have a SYE/CV combo. The differential is shifted slightly towards the passenger side, so the CV helps to compensate for that. You will also need at minimum, adjustable upper control arms. Also, the axle is about an inch shorter in length than the D35, so you'll need wheels that aren't going to rub on the brakes.
The install is relatively straight-forward. I built my own platform to sit my axle on by nailing together four 2x4s in a square. I then mounted casters underneath, and made sure it was high enough to slide a lift underneath. That way I could slide the axle around, adjust it, and lift it by myself. But with two people you should have any problem.
Adjusting the pinion angle is simple. I just used tie-straps to rotate the axle until I got the desired location.
Definitely re-gear the axles to match. You can't use 4x4 without having matched gearing. In terms of lockers - if you're unit has a LSD carrier you HAVE to swap to a non-LSD carrier to use a lunchbox locker. Try to get a non-LSD if you can. Some units also have ABS. I just unplugged and removed the unit, then screwed on a small tab of metal into that space with gasket sealer.
The 8.8 is also a c-clip style axle. The c-clips are STRONGER than the D35 so you don't typically have any issues losing a shaft. But, if you want to pull a shaft you have to unbolt the carrier pin, remove the c-clip and than pull the shaft. It's straightforward, but when you undo the 8mm screw that holds in the carrier pin, soak the bolt in PB Blaster for a while, maybe heat up the bolt, and use a box wrench to loosen the bolt.
Many times this bolt is seized in your carrier. If for any reason that bolt snaps or breaks you're s*** out of luck and you're left with little option but to cut the carrier.
I would recommend when you first receive the axle to sit down, open the differential cover, locate this bolt. Using picture below, that black round bolt facing --> is the 8mm screw you have to remove. Before doing anything else or before trying to have to loosen it while it's installed, just do it beforehand and then retighten it yourself. This will save you a headache in the long-term.
Any questions you may have, definitely feel free to message or ask me. I love my 8.8 axle and have worked extensively on it. The only thing I haven't dived into it setting the ring and pinion backlash.