I'm not sure what spring you are talking about. I am certainly glad to look at everything. After it happened twice, I took the whole rear end off. I suspected a problem in the drive shaft, but it is in good shape. The rear splines were in good shape, but they were a little dry, so I lube them when it happened the first time.
You're right about it being a dangerous problem. These 2 times it only happened when I got off, but I worry that I may think I'vefixed it, and then I hit a hard bump, and it comes off while I'm riding. Both times, the shaft has cocked sideways and locked up the back wheel.
I seem to have a knack for having non-routine problems.
Thanks for your help.