36 ford pu Transmission problems?

[jers36]

I have a completely stock 36 ford pickup the trans # and frame # are the same. I was backing it out of the garage the other day and when I put it in low gear to move forward the left rear wheel completely locked up and it would not move forward, I put it in reverse, and it moved backwards ok then again in low & it locked up again. I then put it in 2nd gear and was able to drive forward into the garage. Sense then I put it on jack stands with the back wheels off the ground and started it and ran through all the gears and everything shifted and ran through all the gears just fine over and over again. Today I backed it out of the garage again and it locked up the left back wheel again and would not move until I put it 2nd and pulled it into the garage again. Does anybody have any Idea what is going on? Thanks for any ideas