I just picked up an '86 mustang, and i managed to get it stuck in 5th. It was in N, and I reved it and it caught 5th and stalled. If I put the shifter in N, it will drive in 5th. If I put it in any gear, it binds and stall. Tranny is a 4 month old reman, from suncoast according to the PO. Any ideas? Im thinking syncros, as they all look the same, but have different taper based on the gear.

I would look more at shift forks and the shift tower,
5th is in the tailhousing so you can get at it in the car
pull the shifter to make sure it isnt fubar
then pull the tailhousing to check out 5th and its fork