I have an 88 and a 4.5" lift on it. I have done no mods to the driveshafts or to the t-case and I have no vibration or funky stuff going on because of the lift. The angle on my rear differential was changed with the springs I put on. The front angle was changed with the upper and lower control arms (adjustable is the only way to go).
You will need to replace a lot of things when you take an XJ that high though. Upper and lower control arms, trackbar, swaybar connects (go with disco's, I like JKS). Some of these may be included in your lift kit others may not but you will need them. You may be able to get away without the upper control arms but probably not. Make sure that you take off the rear anti-sway bar. You won't need it back there and it will cause a lot of funky stuff in the ride of the XJ if it is there and lifted.
I put 32" tires on mine and had to cut the wheel wells and I had to add to the bump stops. There are other obvious things like brake lines, shocks and springs that you will need. Those should come in whatever kit you are looking at. What kit are you considering?
I can explain this in more detail if you have more questions.
I know, I know this is a newbie question but i am going to ask it any ways 
