Does the engine rev to max rpm as if in neutral (when in forward) or does it seem as if it's fighting the brakes? If it revs to governed rpm's then check your directional microswitches (either in the handle assy if there's a shift lever or in the gas pedal if monotrol) & check the directional solenoids on top of the trans.

If it doesn't rev to governed rpm's you may have broken return springs in the wheels. May have to pull the brake drums & have a look.