My water pump went out. When I was replacing it I noticed the oil gallery gasket was blown. But, I never got a low oil pressure warning. It must have been on the line. I changed the water pump, gallery gaskets, timing chain and harmonic balancer. Removed intake resonators too. No change in the vibration. My theory is the low oil pressure all those years wore out the main bearings causing slop in the crankshaft? A way to test this theory would be to dump a bunch of Lucas oil stabilizer in the motor, like 3 qts, see if the vibration mitigates. I ran 15w-50 for one oil change. It seemed like the vibration was less for a couple days, but then came back. I would try the stabilizer myself, but my G got totaled and is gone. Sad.