If it will only spin in Rev, when in put reverse, then Rev gear is dry and hanging up, thus it won't spin freely when in neutral.
Went through this with a friend and an old dry, rebuilt toploader. It would only move when in 2nd. Put it in any other gear and it was locked up. I pulled the cover and found 2nd gear brass was "stuck" on 2nd gear. A quick flip with a tip of a screwdriver to get it unstuck from the gear and it was fine.