transmission trouble
 

I jsut bought a 1991 trooper that has an issue with the trans. The previous owner didn't have the cash to fix it so I got it for a decent price. Anyway when you start it up and put it in gear it goes very well in deive, reverse, and all the lower gears. Then shortly afterwards it will limp along and then just rev going no where. Turn the key of and re start the motor and the same thing happens. The previous owner said it might be as simple as a transmission filter. I don't know much about these vehicles but I figured I would take a chance. What do you all think the issue is?

Thanks
Ryan

I was digging around the web and found some guys who had 94s and 95 troopers who said there is a little module ontop of the trans that can go faulty. Its a $100 fix. They were complaining about slipping gears similar to mine. Any thoughts?