You probably should change the filter. My '97 Trooper has 84k on it, and I opted not to. I've really never seen one plug, unless the trans had a significant failure causing lots of debris to reach the filter. I'd use parts store Dextron III, as there's not much difference in any of them, except a synthetic. Stay away from synthetic in a GM overdrive auto. The ONLY additive I would suggest is Lubegard in the red bottle.

http://www.partsamerica.com/Product...;mfrcode=LBG&mfrpartnumber=60902

The rest are at best a waste of money, if not downright damaging. Do a search on the www.bobistheoilguy.com forums under trans fluid for this additive. This is NOT a snake oil. You can find it in many auto parts stores, but the funny thing is that it's not out on the shelf with the auto trans snake oils. At both Advance Auto and Autozone around here, it's behind the counter.

EDIT: Since you ARE having a slip problem (potentially), I would definitely go through the trouble of pulling the pan and replacing the filter. What you do or don't find in the pan and/or filter will give you indication of the state of the trans.