Clunking, surging when I press clutch pedal to floor

I have a 99 XJ w/ a 4L, 5spd, 4.56's, 8.25 & Powertrax in the rear. Sometimes when I press in the clutch to the floor when coasting at a slow speed with the vehicle in 1st gear, the vehicle makes a clunking sound and jerks a bit forward repeatedly.

I know it's not the locker because this has happened before the time it was installed. When I shift, I make sure I'm not shifting fast because if I were to, I would probably grind on the first 2 shifts. I make sure the rpm's are dropping before I shift and I try to be gentle with her on road.

I've had the XJ since 275,000km, (125,000 miles) and today the odometer reads 334,419km. I have not done the clutch since I have had the vehicle, but I am thinking that this is the reason why I am having this issue.

My questions is, has this happened to anyone else with a simular year/model as mine is and if there is, was there any transmisson work required aside from the clutch to fix the issue?

I had this happen 2 times for 2 differnt reasons first was a easy diagnosis clutch wasnt fully disengaging. I ended up having to replace the tranny the same time since it took a toll on the bearings and stuff. the second time and most recent I figured out it was the input on the transfer case wobbeling (got progressivly worst in a matter of hours) and now Im in the process of replacing the t-case and trans again.

Just sharing my experience hope yours is less costly