1999 Chevy Suburban Driveline vibration

I just purchased a '99 Suburban K1500 with 121,000 miles on it. I drove it around the neighborhood before I purchased it; but didn't take it on the highway. After I bought it, I noticed a vibration when driving from 60-65 mph. Tried changing gears to induce vibration at a different speed; but vibration comes at the same speed in 3rd as I do in OD. Had the tires rebalanced and the rear drive shaft replaced (wanted greasable yokes). Still get the vibration.

I noticed in the stack of receipts the former owner gave me that he had the output shaft seals replaced under warranty several times, and finally had the output shaft replaced (also under warranty). The output shaft still leaks. Could the shaft be out of alignment and causing the vibration? It's a clean truck; but the vibration is really annoying. Help!