Before you take a look at the tranny, have you looked at the servo that does the shifting?
(if there is one)
If there's a servo that does the shifting, or something that controls the shifting I would bet that it's just not engaging all the way.
What engine are you running? If you are running stock and are not overly abusive on reverse the gears should be fine.
If you do need to or decided to take a crack at the tranny be prepared for a long haul to get it bakck together again, lots of spinning synched up stuff in there lol.. no fun for me..