Valves are clogged. Does the fluid smell burned? Try changing the fluid and filter, and I have always had good luck with Lucus transmission fluid "fix" even though I have always used it in transmissions that weren't slipping. The Lucus stuff claims to repair slipping transmissions (most likely by removing the deposits that form near the orifice of the valves). Since a transmission fluid/ filter change only changes 1/3- 1/2 of the overall fluid (most remains in the torque converter), after 500 miles change the fluid/ filter again.
When my bro-in-law had a similar problem in his old Neon we did this and it fixed the slipping. I think based on the year your Jeep would take Dex/Merc III, and if it does I would not recommend ATF +4 since it is a sythetic and XJ guys say it ruins the clutches. XJ guys swear by Castrol conventional Dex/ Merc (says III in the print on the back).
DO NOT HAVE THE TRANSMISSION FLUID FLUSHED, IF THERE IS ANYTHING IN THE TRANSMISSION THAT SHOULD NOT BE THERE, A FLUSH WILL FORCE IT SOME WHERE IT DOESN"T BELONG.
Good Luck and keep us posted.