92 K1500 automatic - ignores "2" "1" gear
 

This is a problem i inherited from the previous owner, so I don't know when or how it happened...

Truck drives fine and everything. But if I try to shift down to "2" or "1" nothing happens. i.e. the transmission does NOT shift into 2nd and stay there. It acts just as if it is in "D" still (well actually O-d "(D)" ). Slowing down or speeding up doesn't help. It will shift up or down as it pleases.

It is annoying going down big hills. In "1" the truck will happily coast from 0 to 70+mph downhill. Is this an easy fix?