Ford Repair: Rear Vibration, rear axles, driveline shop

Question
Hi James. I own a 2002 Ford F150 super cab short bed 4.6 l engine. 2wheel drive. It has 84,000 miles. I have had a vibration in the rear for as long as I have owned the truck. I am finally in a position to repair. I have changed all brakes and rotors with Wagner Thermoquiets. I have had the Driveshaft balanced at a local driveline shop. A slight bit of weight was added to the rear of the shaft. The vibration is felt in the seat and floor of the truck when driving between 36 and 42 mph. Coasting does not change the vibration. The vibration then returns at about 55 and above. It is very annoying. I have also replaced the shocks and have new tires BFG Long Trail TA Tour and they were road force balanced. I recently reindexed the driveshaft with no real change to the vibration. There are no leaks from any seal such as the pinion or the rear axles. The parking brake is loose. I am stumped as to where to go next. Some others say to replace the drive shaft. I have the single driveshaft that is about 72-75 inch long. How much would it be to replace the driveshaft if needed? Or can you think of anything else that I should check?

Answer
There are a few possibilities that I can think of. The first is the wheel itself. However, that should have shown up when the new tires were installed. Have the tires rotated just for arguments sake. If you have had the drive shaft balanced, I believe a problem would have shown up then. Does the truck have a suspension lift? The last possibility is a defect in the rear differential.