Busted Tranny?

Hey, I have an M6 Z28, no problems there, but our other car is a 99 Grand CHerokee Limited (V8). The transmission is shifting hard from 1-2 sometimes. Most of the time it is fine, but sometimes it will shift and u can hear it thud, and its so hard you feel like you got whiplash. I figured either way this isn't good, but like i said most of the time its ok. Im also debating whether or not to do a transmission flush.

Can't say too much about the Cherokee since this is an F-body site. But, I'd recommend atleast checking the fluid. It's color and smell can give off a lot of information. If it's not cherry red and smelling "normal" I would recommend a flush for starters. If it's brown or discolored and smells burnt I bet that's part of your culprit. Hopefully your only problem. How many miles are on the vehicle?