[ozi jim]

It may be a clutch issue.

If it has been sitting for a long time the clutch plates stick to the flywheel.
a few months is not that long though.

Question
If you start it in neutral is it fine or does it still want to drive.
If you can start the eng in neutral and gently try to engage reverse and it grinds the clutch is not releasing.

If the clutch plate has stuck to the flywheel I have released them by foot on the clutch, foot on the brake (toe and heel) start it in gear and try to release it by eng power.
Answer the questions first though.

If it where a box it would be 3rd - 4th syncro hub or input to output shaft to output shaft bearing are likely possibility.
If the selection feels normal it is probably ok, when they break rolled pins or something like that it feels to easy or to hard to engage.
If it does not want to drive away in neutral its a clutch.

jim