truck rolls forwards, but not backwards

[redstone65]

There are many mysteries on this '70 model f-100 I just bought. But, the main concern at the moment is that the rear wheels will not roll backwards unless the driveshaft is removed. I set the tranny (automatic - not sure of model - C6?) shift lever to what should be neutral but no dice. I read some things about possibly a reverse band being too tight.

Any ideas spring to mind on how best to troubleshoot this?