Originally Posted by MarkNog
If it didn't cause vibrations before, why would it be causing it now?
Because the differential on an 8.8 is offset to the passenger side; it's not exactly in the middle like a D35 or D44. Because of this, it's almost impossible to not get vibrations without a CV drive shaft. Even those who have an otherwise stock height Jeep have needed to get an SYE/CV shaft and all that entails. If he has any amount of lift, this applies even more.
OP, you might end up needing a CV shaft; most anyone who has done an 8.8 conversion has had to get one, so I would expect to need one eventually. You'll thank yourself later for getting one. If you truly need that 8.8 for the offroading you do, you're going to want a CV shaft.
That being said, before spending money on an a CV shaft and all that entails, try this: I assume you used a flange adapter to bolt your shaft up to the flange on the 8.8, correct? Try having your shaft rebalanced with the flange adapter. After having a very small vibration even with my CV shaft and then talking to a local drive shaft shop, apparently it is very common to have vibes with the adapters.
And before any of that, triple check your pinion angle. If you have the stock shaft, it shouldn't be in-line like a CV shaft would. Get an angle finder, and set it properly: