Drive until it fails....if it fails!
While you might find someone that did have preventable damage with a 5th gear failure, the majority of the time the nut just backs off and 5th gear goes away as smooth as pushing in the clutch.
I know 96 and 97 trucks that have never pulled anything and lost 5th. I lost mine 3 times in my 96, inbetween 2nd fail and 3nd fail, I paid to have a "fixed forever" nut installed. My truck pulled lots of long heavy loads and 99. 9% of the miles where in 5th. I replace with Dec 2000 brand new transmission.
If I was to by another dodge 5speed (even pre98), that was all factory with no updates or transmission fixes, I would drive it uptil a problem occurred. While their are much better (maybe even permenant) fixes available now, no need to fix something that might never break. If your setup to do it yourself, have at it. If you have to pay someone to fixit I would wait!
NOT EVERY DODGE DIESEL LOSSES 5th GEAR!
jjw
ND