A little tranny help, please?

[81 Redhead]

Just bought an '81 C10. It runs decent. Only trouble I have now is that the automatic ain't automatic.
It'll cycle through the gears if you start out in L1, then shift to L2, then to D.
Starting from a dead stop in D, the engine winds up but the tranny won't shift.
PO sez the tranny was rebuilt just before he bought it. He put about 300 miles on it (it's a farm truck), then parked it. It sat for nearly 3 years before I got hold of it a couple weeks ago.
I'd really appreciate any input or ideas about what's ailing this thing. Thanx in advance!