Welcome to Allpar. Rule out the cheap things first. Check the shifter cable adjustment to make sure that 5th gear is being fully engaged.
Change the transaxle fluid if it has never been done. I use the Mopar T-350 fluid. It holds ~2.5 qts.
If it is deemed to be an internal issue, the transaxle will have to come out and apart and possibly have the 5th gear actuator fork or synchronizer assembly replaced.
SLIPS OUT OF GEAR
Transaxle disengagement may be caused by misaligned or damaged shift components, or worn teeth on the drive gears or synchronizer components. Incorrect assembly also causes gear disengagement.