try swapping the fwd and rev solenoids and see if the problem reverses itself. If it does then you have a bad solenoid valve. If not then the problem is internal to the transmission control valve. One of the valves in the control valve is stuck or a broken spring or something. You will have to physcially remove these and inspect them for sticking, dirt, or broken parts or wear.
you can also just remove the wires from the solenoids and it should just sit there in neutral, if not then like i said, start in the control valve and go from there.

It is possible there is an internal clutch issue but i would check the things i mentioned first before tearing the tranny apart.