89 Blazer Full Size 4x4 with a vibration

I get a vibration starting at about 50 mph, getting significantly worse by

60 mph and continues for as much as 85 mph (haven't gone any faster than that). The tires have approx 15k miles on them so I got a wheel alignment, all tires balanced and rotated with no change. The rear tires had some cupping and couldn't be balanced so I replaced them both, but still have some vibration although not as much as before. I've recently changed out both rear u joints, had the rears brakes done, with new bearings and seals installed. With the truck up the hoist and the rear tires raised there is no vibration, rear drive axle looks good, and both rears tires spin smooth. The tranny was rebuilt about 4500 miles ago but the torque converter was not redone. Could it possibly be the torque converter? transfer case?