1983 528e Transmission Issues

As I was driving today, the transmission (manual) started popping out of gear and wouldn't stay down. I tried stopping and re-engaging the transmission, but it kept popping out and grinding like it does if the gear isn't properly engaged.

I just bought it a few months ago, and before that it was in storage for 2 years. I'm really hoping it just needs some new transmission fluid, but can anyone tell me if this is a sign that the tranny is dying?

Any help at all would be greatly appreciated.