I had a similar vibration a while ago and It was a u-joint. Had both replaced and driveshaft balanced. Vibration gone. My rear view mirror would vibrate to where everything was blurry. Guy showed me the old u-joint had a spot where it would kind of stick when you would spin it. Not sure how you can test them without removing them. Had it done at a driveline shop. Cost somewhere north of 200$ for everything.
I could have written that. Had the same issue, with one more complication.
The original driveshaft went to the driveshaft shop locally, the rear U-joint was completely wiped-out. However, the shaft was also bent. They claimed to get "most" of the bend corrected, and replaced both U-joints (although there wasn't anything wrong with the front joint.)
Before, noticeable vibration from ~68 mph, worst at 73 mph, gone by 80 mph.
After, dead-smooth at all speeds; even fixed a minor vibration I had starting at 25mph that I never noticed until it wasn't there any more. I had assumed it was rough road shake.
This lasted about two years. Vibration came back exactly like before--worst at 72--73 mph.
Both U-joints were still good. I grabbed a different driveshaft, replaced the rear U-joint in it, and fixed the vibration again. Same story--cured a lower-speed vibration I didn't know was there until it wasn't there any more. Apparently, the original driveshaft bend got worse again.
If this ever comes back, I'm going to have a Trailblazer EXT driveshaft shortened to fit my short-wheelbase Trailblazer--the EXT shaft is much larger in diameter.