My first thought is to check the transmission fluid level. After its warmed up with at least 20 min of driving. With the parking brake set and wheels chocked. Place in "neutral" and check fluid. it should be at the top of the hatched area or full. I have found simular problems due to low fluid level. I would normally find them 1qt or more low. Most of them checked the fluid in park position. In park position no fluid flows thru the valve body. In Neutral fluid flows thru the valve body and oil cooler.
[FONT=Trebuchet MS]01' Jeep TJ (Jeep #5)
3.5 RE Custom SuperFlex, Raingler, Kilby, Currie, Banks, Borla, American Racing, Custom mods, Always more in the works.