The 4 speed auto uses the old 3 speed Torqueflite as its base. In Drive, the FWD clutch engages, and the mechanical one-way clutch at the back of the trans holds the low gear drum. If the mechanical clutch doesn't work, the thing just spins freely. If you pull the lever to #2, same thing. If you pull the lever to #1, the rear band engages hydraulically and holds the low gear drum, and away you go.
My old-time education from the 80's says you have a bad mechanical one-way clutch, which is an unusual failure....those things are usually very reliable.
Was there any metal in the pan when you changed the filter?