Before you spend 2 or 3,000.00 on a rebuild try a 16 oz. bottle of Transx in the yellow bottle. There is a 32 oz. In blue bottle don't use it use yellow bottle usually about 14.99 in parts stores.
Many times slipping or even won't move or will backup but not forward or vice versa is due to stuck plungers in the valve body of transmission. When you add crank car up and let run 10 to 15 minutes then put in every gear several times with brake applied…
I have a friend that has seen many start pulling and shifting perfectly…