you have to z the frame to lay flat on any wheel combo. with 20s you have to move a lot of stuff. its been awhile since i did mine, but new air intake, moved the bat, fuse box, computer, threw away the wiper fluid thing, maybe more. for 22s (and maybe 20s with a z) you have to do something with the hood hinges, they will hit your tire and you cant open your hood when its laid out. the reason everyone narrows the rear about 3 in is so they will only have to buy one new axle. (one side is longer than the other)
for a fuel cell, you can buy this
http://www.genright.com/ProductInfo.aspx?productid=APR1000. the one for the 97-04, you also need the lock ring for a 97-04 jeep. you just install that on a 12in deep fuel cell, and your stock pump and sending unit will work. if you decide not to narrow, you need at least a +40 offset. check out
www.alldakotas.com