As I recall the fifth gear is the easiest to fix it is in the back half of the case, so you wouldn't have to take the main gear box apart. syncros sound like the problem. I am not a trans guy but my dodge has been in the shop 3 times this year for a bad syncro ring (machine error in syncro)
Let us know what you find. either way you are gonna be dropping the trans.
GOOD LUCK
