Transmission grinding in reverse

Just bought this car for our grandson to go to college and the driver side/left axle needed replaced so we did and when he was backing it out of the shop it made a loud pop and then started to grind and rolled back in to the shop. So grandson started to remove the transmission so I helped him remove it. We pulled one from a salvage installed it and it is making the same grinding sound. So we went ahead and change the right axle too. But now all it does is grind in reverse. Took the new axles out and put the old ones back in and now it still grinds.
I am very puzzled.