Had the same problem and spent $2500 for a new trans just last month. I found out that the tourque converter was bad. The fins inside were coming off little by little and mashing into the trans pump and all around. Mine thou didnt want to go over 20 miles an hour when it was hot and kept popping out of drive. If you have high millage on the truck i.e. 85k, then I would take it to the dealer or a good trans shop. Changing the oil could just cause more problems.

[color:"red"] 2001 Trooper 3.5L [/color]
Got a KN filter, finally...
2" Indy suspension lift
275x16" tires