I`m not a YJ person, nor do I claim to be one on TV....,but Ive never seen a vehicle with two brake lines per wheel going to one caliper...do you have 2 pistons per caliper? If so, call up the lift kit guys, and tell`m- they`ll need to know.
on all the lifts Ive done, the shims always went in the back (that came with the kits) to help with normal 2wd driveline vibration (hence dropping the transfer case will help solve that as well) that the jeep will see most of--
I have seen very,very,very few shims for front applications....but they do exist-you`ll not know you need them untill your done and have it aligned. Its not something thats regularly done--
with the shims in the rear tilting the axle up, you`ll reduce the rear pinion angle-- the theroy is that both angles need to be the same (transfer case/driveshaft), or as close to the same as you can get to reduce/eliminate vibration-- your going to have it anyway
--- by shimming the rear, you may end up dropping the transfer case to make the angles closer to the same as what you started with-(BUT, you may also not have to-)....unfortunately, by dropping that transfer case, youve now messed with the front driveshaft angle...its a catch22.
The shims help your jeep live in 2wd mode happily-when your in 4wd playing in the mud, or whatever your choice of fun is, your probably going slow, not on pavement...so the vibration from the weird front angle on the driveshaft wont be noticeable....you will however, have to watch all the u-joints more so then normal-- the more the angle, the quicker the wear.
go turn a wrench and get greasy.