Have you changed or checked the fluid already? If not have it flushed and changed before doing the overhaul as it may just need to be serviced. Normally when an AT goes reverse is the first one to go as it's the gear used the least that's according to the AT specialist who rebuilt the tranny on my BMW. You can buy an overhaul kit and have a AT specialist rebuild it for you that way you know that all the parts are new.