[35cab]

I had this happen on a 40 tudor some 15 years ago. Very gentle start from traffic lights then a slight "ping" and no drive.
Towed it home a couple of miles, then jacked up rear, pulled the drum/hub and the end of the half shaft came with it it! A very old grubby fracture and a tiny bit of clean metal, obviously an old fault. I hope this is not the situation in your case and it is only a sheared axle key.
Interestingly I was going through a few half shafts looking for a good one and in one I found the remains of a sheared key wedged into the slot, no distortion on the slot but the key must have been seized in and sheared along its length.
Good luck.