84 Fiero w/T125C slips in reverse

I was wondering if anyone would know why the transaxle in my 84 Fiero slips in reverse but shifts great in all 3 forward gears.The fluid is full and a nice red color and does not smell burned at all.I just got the car running again because it had a cracked head so it has been sitting awhile but I don't know if that would have anything to do with it.Any help would be much appreciated.