will not move in reverse

This would not be an issue in the rear end if the vehicle will move forward. Also, it is most likely not a transmission issue, but this is where we should start.

First, we need to check for codes. Here is a guide that will help with checking. I doubt you will have any, but we need to check.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Next, we need to put the vehicle on jack stands and see if there is any movement in the tires like there is any slack from the gear train before they stop moving, or if they are locked from the brakes holding.

https://www.2carpros.com/articles/jack-up-and-lift-your-car-safely

The way to find this out is to break the wheel cylinders bleeder loose and see if the brakes release. If they don't try removing the tire and hitting the drum with a hammer to see if the shoes will break loose.

Clearly if the vehicle will move forward, the shoes are not seized but I suspect the trailing shoe is stuck in a position where it will touch the drum. However, it is unlikely that this is both wheels so I suspect you will find one that will spin and one that will not when you jack it up.

Please see the info below and let us know what other questions you have.

Thanks