When you spun a wheel did the one on the other side turn the oposite direction? If so, things are normal, and your differential appears to be okay.

Drop one rear wheel back on the ground, and if your transmission is stuck you should not be able to turn the wheel that is still raised.

1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more...
It's true. I really don't do anything but browse the Internet looking for trouble...