Keith, the thing about the front is that it's not how LONG of a d/s you'll need, but how far it'll compress. I'm on 6" of lift with a completely stock driveshaft. I've also got longer UCAs and LCAs to keep the wheel centered in the wheel well.
Take a look at the suspension from the side. As your suspension extends, the front axle will pull back towards the transfercase. This is why compressed length is important, if your d/s won't compress far enough, you'll pound it right back into your transfercase.
As far as stopping vibes........ that's another story. The stock front d/s is a single u-joint at the axle, and a constant velocity style at the tcase. This means that the normal operating angle at the lower u-joint should be zero, or straight. As you lift an XJ, you want to keep your caster in the 5 degree range, and as a result, the lower u-joint now gets a few degrees of bend. How do you get around this? Pull the front d/s, put offset balljoints in, cut the Cs off and rotate them, etc. There are quite a few options.
BTW, at 6" of lift, I'd definitely suggest replacing both upper and lower control arms. If you're still running stock UCAs, your caster is going to be all out of whack.
