when you say "brake release seems fine" can the drive wheels turn if you get the unit's drive wheels in the air? can you turn them by hand when the unit is turned off with the drive wheels off the ground?
have you checked the inching spool on the transmission linkage to make sure it is releasing and not binding? can you further describe; "seems to want to move but just comes up short" is the trans slipping (motor rev fine like in neutral?, or motor loads up like trying to push down a wall?)
sounds like it's time for a transmission rebuild...