[Eyegore]

Your desribing exactly what happend to my 82. 1/2 way down; grind, roll up; grind. The first time I encountered this was on my 86 burb, just like BigRed's, rear drivers side door. Went to a wrecking yard that replaces glass, he said it's common for the motor to go out with the symptoms described, rarely he said does the regulator go out.
Do a test, you'll have to pull it apart anyway even if its the regulator. Pull the motor off and bench test it using alligator clamps to the battery. Let it run up, you'll here a grind, then reverse the clamps to roll down, grind. Then replace the motor.
I know its common to expect the worse, but you can pick them up at a wrecking yard, motor & regulator.