What is vehicle model year and engine? This determines the particular transmission used. Have you replaced the transmission and the same slipping behavior is still occurring or are you contemplating replacing the transmission as a fix? I will respond to the older post and relate information which should help with your similar issue.. . . my truck is doing exactly the same thing, did you ever find out what the issue was? I just put in A new transmission and its doing what you described . . .
You have the Chrysler A500 / 42RH transmission in your vehicle. It is hydraulically controlled and hydraulically shifted. The only electronic control is the overdrive engagement solenoid which is controlled by the PCM (powertrain control module) After engine temperature reaches a minimum threshold and other speed conditions reached, the PCM energizes the overdrive engagement solenoid circuit. This allows hydraulic pressure in the transmission to simultaneously disengage the direct drive clutch and engage the overdrive clutch. The override switch on the instrument panel opens the circuit so the PCM will NOT activate the overdrive engagement solenoid. this keeps the transmission in 3rd gear at highway speeds.
There are 2 bands and multiple clutches in the transmission. The attached chart shows which elements of the transmission are in use with different positions of the transmission gear selector and actual gear utilized in the transmission. Adjusting the front and rear bands will not have an effect on this slipping in 4th gear / overdrive. From the chart you will see that both the front and rear bands are NOT engaged in 3rd gear or 4th gear. So the bands are not the source of the slipping. The front and rear clutches are both engaged in 3rd gear and 4th gear so they are not the source of the slipping. So that leaves only the 4th gear overdrive clutch as a possible culprit along with low fluid pressure in the overdrive clutch circuit.
I believe the overdrive housing can be removed with the transmission in the vehicle and that serviced separately? You would need to find a knowledgeable transmission repair facility and determine if that is a viable option.