More than likely a drive shaft issue if you can feel it's not in the wheels. Have you had the DSHAFT out recently? Sometimes if you bolt them up 180* out the balance is thrown off and they'll rumble. I had it bad in my fox body that had one of those stock rubber dampeners on the front yoke that was torn up. (not sure if SNs have the dampener). Could have smacked the DS on something? Get under there and check it out.
Anyway... I solved my fox body rumble with an aluminum DS. They are worth the money for a nice one. They free up some ponies and drop un-sprung and spinning weight which is a very good thing.
