Well, if you have ruled out the clutch...and there is no grinding, which would mark a bad gear, hub sprocket or wheel sprocket.
Then I am afraid the OD is next. You very well may be stuck in OD, and you can damage the OD sprag clutch trying to go in reverse. There is no reverse in OD, so the sprag clutch attempts to prevent reverse movement of the car. If forced, it will shatter the sprag. Most who have tried to reverse with the OD stuck describe reverse as "hitting a curb" feel. If you are lucky, your sprag is slipping, rather than locking and shattering.
I used straight non- detergent 30w in my OD, as the book recommends when I rebuilt it. The OD would stick engaged until I took a wrench and wacked the OD bell a few times. I switched to Red Line MT90 and the sticking immediately stopped. So...
Try tapping the OD bell and listen for a "ding" as the OD ring snaps back. If that does not help, then check the solenoid adjustment to make sure it is allowing the OD to disengage. Finally, make sure you switch to Red Line RT90. I think one of those will get you going.