So Sat. I pulled the drive shaft, closely inspected all U-joints, and felt for slack in the diff. All checked out OK, but I did notice the yoke could use some grease.

I went ahead and let the dealership bend me over for the $45/pound can of GM "Special Lubricant". Whether or not it is worth it, I guess I'll find out down the road, but it solved my driveshaft noise problem for now.

Guess I need to get my sound effects right...must have been more of a "honk" than a "clank".

Has anyone ever used any other lubricant for this particular slip yoke noise and had any success?