Put it into first gear then unclip the forward cable then push it fully into first to make sure it is in, then jump in and attempt to drive it hard in that gear (bearing in mind it's in 1st so use the clutch to start it, If it pops out then it is the gearbox, you can drain the oil into a container with kitchen roll in and you'll see a metallic sheen in the oil, I'd expect so anyway as I'd be wanting to see evidence of wear from the gear components or synchros(generally speaking).
You can try new oil but at 50k i expect if it hasn't been damaged etc then it should have the original oil in and that shouldn't be the problem.
50k or indeed from new as in the other members van seems a bit silly in this day and age but i guess engineering or whatever isn't something we care about anymore