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Under the mounts. A block of wood and jack will reveal if its sagging which in turn will throw off your driveshaft angle. If you rebuilt everything and it was smooth as silk but now has the same vibration 4 years later, that doesn't mean you can eliminate the rear end from being the issue again. Have you removed the cover and inspected? Look for metal in the oil, pitting in the bearings, check gear tooth contact pattern.
The bolt should be 1/2" x 13 TPI x 3-3/4" Grade 5. With all of that oil, those upper saddle mounts maybe getting soft. Locate and repair the oil leak, replace both upper saddle mounts and the missing lower isolator and plate.
Urethane is impervious to petroleum oils. IMS, the plate is integral with the factory style rubber isolator - it's one piece. At the very least clean and check the upper mounts for swelling and firmness. If they are the original mounts, they are probably toasted.