Is there some way to test a gearbox when its out of the car?
fraid not
That's no true Matt, you can pull it apart while not in the car
One thing you can do is put it in all the gears and spin the drive shafts and see how it sounds.
I had 2 $25 luckydip mystery boxes here I did that with.
one was quiet as a mouse in every gear and when I put it in my bug drove like a brand new one.
the other was that one I pulled from the home made mower and made horrible gratey sounds like it had sand instead of oil in it.
syncros were f**ked and you couldnt hear the engine over it but it made a good exchange core for a rancho box