The rear wheels will not turn in reverse at all. I

Rear wheel drives like this have pinion gears with rachet pawls that allow them to go backwards freely while locking when the transmission drives them.

You need to take off your rear wheels and carefully remove the pawls. They look like little squareish metal shaft keys with springs under them. You should notice that the keys or pawls are covered in what looks like grease.....which is actually worm metal from the pinion gear and oil mixed together to form this pastey greasey substance. Clean all that stuff off and lightly oil *( light oil...3 in 1 kind of oil) and reassemble.

You should hear the nice clean clicking of the rachet pawls as you roll backwards again!

Good luck and be careful!