Any idea what could cause this???? 86 k5 vibrating

[KaptinK5]

My 86 Chevy k5 blazer has had a really bad vibration lately and i cant seem to figure out what is causing it at this point. i had run low on gear oil in the rear diff due to a very slow leak i was unaware of and that was the first thing i checked (vibration started some time after this). All of the gears seemed to be in good condition. I then found a bad U-joint and replaced that but it still vibrates. it only does when you accelerate over 30 and once you let off the gas it goes away. Someone suggested the transmission however i don't have any problems with it shifting or slipping so i don't think that could be the problem although i could be wrong. i have no other obvious drive train issues i can find so anyone who can help me out in diagnosing the cause of this would be helping me greatly. thanks.