Vibration while driving? Need help

2WD extended cab Sierra 1500 with a V8. 130,000 miles on it, no towing, no hard driving.

I developed a vibration about 6 months ago anytime the truck was driving over 25 miles per hour. I took it to Goodyear, and they told me that my tie rod ends were bent, and I needed inner and outer tie rod ends both sides of the front of the truck. I paid them to do it. Vibration was just as bad at this point. Took it back, they told me it was the front tires, I had two new tires on the front of the truck. I paid for the tires. Vibration was still bad, took it to another garage, they told me that the problem was two U Joints, I paid them and they replaced them. Vibration MUCH, MUCH better, but still noticeable between 35-50 MPH.

Got tired of vibration, took it to a family recommended mechanic on Monday morning. He called me Monday afternoon, told me that all three U Joints were bad, another bushing maybe on the drive shaft was bad. He replaced all three U joints, the bushing, and gave it a tune up and transmission service. This cost me $1,800. Tuesday when I went to pick up the truck, he told me that the drive shaft was out of balance and that it needed sent out and repaired for another $100-150. Picked up the truck last night, vibration is horrible now, as bad as ever. Called mechanic, he told me "it needs a new rear end." Can someone give me an idea what might be wrong with this thing? I love the truck, its paid off, I want to keep it, but I can't drive a rattletrap, and I SURE can't keep throwing money at it. I can feel the vibration in the steering wheel and in the gas pedal at all times, REALLY bad at 30MPH and up.

I've had it since it had 20,000 miles on it and has never been in an accident

HELP