2001 Chevrolet S-10 Low speed vibration

The universal joints are the likely suspects, but the truck is at the age when sagged springs become an issue too. That changes the drive line angle and reduces how much the needle bearings roll inside the bearing cups. That concentrates all the wear in a tiny spot and can wear indentations into the roller surfaces.

The clue to this can be very hard to detect but the vibration will change as the back of the truck goes up and down over bumps in the road. It will also be worse under acceleration due to the increased load on the needle bearings.

Sometimes you can identify a bad joint by the reddish-brown-colored powder around one of the bearing cups. When it is shifted out of "park" and the wheels are blocked, try to twist the drive shaft both ways. If you see or feel any movement between the u-joint's cross and bearing cups, you've found the bad one. Other times you have to remove the drive shaft to turn the joints by hand to see if one is tight or rough.