1999 1/2 ton Silverado rear end vibration

1999 1/2 ton silverado 5.3 vortec 110,000 mi.

I was getting a vibration from the rear end at ~40 mph and ~70-75 mph. It felt like the right rear tire was out of balance. Took it in for balancing and was told the tire was separated and "checked". Needed new tires. Tires were old and down to were bars so I got some E rated Pirelli 265-75-16's on it, but the vibration didn't go away. Had the tires rotated and balanced 3 times and they even force balance them. Still had vibration from right rear. That's where I feel it's from. Vibration at 70 mph decreased but was still present.
Checked shocks against new set and compression and extension were fine. Kept old shocks on. Removed rear drive shaft and had it balanced. Was fine. Checked u-joints. Service guy said they were fine, but rear one would need replaced in a year or so. Replaced rear U-joint just to eliminate this variable since it'd need it soon anyways. Still had vibration.
Pulled rear brakes and rotors. Left rear was fine. Right rear rotor showed uneven glazing on outer pad area. Light color towards hub and dark glazing toward outer 2/3's of pad surface. Parking brake pads quite worn and shiny surfaces noticed on inside of rotor in 2 spots 180 degrees from each other. Parking brake seemed hung up. Lubed it and got it moving freely again.
Replaced rotors, pads, and parking brake pads. Actuated parking brake several times and tested with rear end in air. When reset, tires rotated freely. Took for test ride. Still vibrated.
Vibration is worst during first drive of the day when ~40 mph is achieved. After 5 miles or so, the vibration reduces but is still present when 40 mph is achieved again.

Suggestions?

thanks,

dave