If the glass moves independently of the regulator then the sliding jaw has probably failed. Typically the glass slips into the door when the sliding jaw gives. If you can move glass a few cm but no more or if it's firmly attached to the regulator then the problem is much less likely with the $10 sliding jaw and more likely with the $200 regulator :/

In my case I was able to rehabilitate the window by helping it up and down a few times. I'm an nth removed W140 owner. I can't see spending money on a regulator for a window I lower once a year and only to check that it works.

Sixto
95 S420
87 300SDL