Hey Hey, I'm still having problems with my xj. I've taken it off road and played around and never had a problem, but when I hit the road and it was just straight driving; I'd hit third gear and bang, it shook the whole Jeep. So if anyone out there has had the same problem please I could really use your help. I was told to check all the vaccume lines, but I'm still not sure where they all are, I may be a newbie, but I'm learning, slowly but surley. P.S. the snow is really starting to fly here, and a 4X4 sure would be nice. Thanx.

steve o try to check your tranny fluid. my truk did the same thing, the trans fluid was low. hope this helps

It could be your transfer case. My '84 gets this a bit, if the vaccuum is low for a long time, such as when you are accelerating, there is no vaccuum to hold the transfer case shift lever in high range. It wants to go to the neutral position. So it will jump in and out of high range and make a hell of a jolt.
There are one way vaccuum valves on the vaccuum lines to the transfer case to prevent this It may be that one valve is leaking or the whole line is leaking.