The drone could just be old age or low oil in the diff, but the vibration could be a lot of things. If you've already done the U joints then I suggest you put the rear up on jack stands and work the axles for possible play at the diff end (could be diff bearings) or more commonly the hub end, because these cars are prone to wearing out rear wheel bearings. Spin the wheel and work it up and down to listen for grinding & any play. The wheel bearings aren't too bad of a job if you get to them early - just need someone to press the old ones out and new ones in. Think I last paid $30 for the bearing and $20 to a local garage with a press.