Ford - F150 :: 2005 - Rear End Vibration

So I have a very annoying vibration in a 2005 Ford F150 FX4 5.4L w/ approx 122k miles. This 'rear-end' vibration became noticeable about a year ago when I put a new set of tires on. And just for the record, this vibration was never a problem beforehand. So it should be obvious that I have bad tires right? Thus, I instantly go back to tire dealer and explain the situation. They took it apart and checked balancing on all tires and confirmed that they were fine. So after dealing w/ it for a few weeks and rotating the tires around a few times and experimenting with pressures nothing worked. This time we noticed the U-joints were now loose on the driveshaft, which we then replaced. But this didn't fix the issue. I then took it to a separate Ford dealer to have the tires 'road force' tested, which nothing found.

I then had Ford dealer try to diagnose the issue with several trips back and forth with their apparent 'best' people on it. I went on numerous test drives explaining the issue. The vibration only seems to be noticeable on road and with speed. Not in the shop in the air. It seems to be pinpoint approx. 50-55mph. and minimal at lower speeds. Also seems to get worse with turns and bumps in the road. You can literally watch the bed of the truck vibrate in the mirrors. (also seems to be more on drivers side) It doesn't affect the steering or anything in front of vehicle.So Ford also had me come back another time to use a special vibration analyzer on it, which still found nothing. They checked ball joints, suspension, you name it. The only fix they could recommend was a new driveshaft, which I regretfully accepted the challenge and now it's sitting in my garage. It didn't work. I ended up taking my original shaft to a balancing shop and they found no issues with it. Also looked at front/4x4 shaft, however, it doesn't engage with normal driving 4x2 so shouldn't be the cause. Approx. 6 months later, I went back to same tire shop and had them exchange tires with a completely different set (factory recommended size LT275/65/18) thinking it still had to be the tires- no fix.

So then on to other things: I started with changing out the rear differential fluid- no fix. I also put on new rear brakes and rotors- no fix. I put on new rear shocks- no fix. I also took off the parking brakes and inspected all cables and housing, which could apparently stick sometimes- no fix. I cleaned all contact areas of wheels and rotors onto the wheel studs to be sure there were no abnormalities- no fix. I even took off the front 2" leveling kit and tried driving without, no fix. (But this was never a problem before.) Keep thinking it could be something more deep in the rear-differential, but most reports say it would be very rough and noisy/grinding-which it isn't. So after probably 6-7 trips to at least 3-4 shops and perhaps $600 later I now have newer tires and a spare d-shaft, which Ford won't take back by the way, and I still have the vibration.