Hayapower, I believe they are the original U-Joints so they need to be changed anyway. I ordered 3 of the MOOG/PRECISION 270 U-Joints they are the Greasable Super Strength models, but that center carrier bearing has different size options which is making it difficult to order without pulling the old one off and specking it's dimensions.

Hayapower, I just tested the Fuel pressure.
When I turn the key on I am getting 36 psi on the fuel rail.
When I start the engine and let it idle I am getting 30 psi on the fuel rail.
When I advance the RPM's to 2000 I am getting 29 psi on the fuel rail.
When I disconnect the vacuum fuel regulator I am getting 39 psi on the fuel rail while idling.
When I advance the RPM's to 2000 with the fuel regulator disconnected I am getting 40 psi on the fuel rail.
When I checked the amount of Vacuum at the fuel regulator connection I am getting 18.5 in Hg vacuum.

I plugged the regulator vacuum line and drove it and just like clockwork when the truck reached 35 mph it started to vibrate.

Hayapower, I crawled under the truck and grabbed the driveshaft and yanked and pushed and jerked it in all directions at each of the U-Joint connections. The only one that I could get to slightly move at all was the one coming out of the transmission. It's difficult to check that center carrier bearing since it is in that metal housing with that rubber dampener. I guess that carrier bearing could be worn out and you wouldn't know it by jerking around on it? I have U-Joints coming in, so I will be replacing them anyway.

Hayapower, I Changed out the Crankshaft Position sensor and also took the Truck to Discount Tire and had the tires inspected and balanced. The Truck still vibrates when It reaches 35 mph. I can't hear any noise that would indicate a converter shutter nor any fluctuation in RPM's as it enters the vibration zone. I should have the U-Joints and a new Camshaft Position sensor by Friday. I'll install them this weekend and let you know how that works out.

What keeps coming up is the vib, in neutral, no load applied,, so it would seem that the rotating components aft with no load would be suspect.
Most often, a U joint/s will make squeaking noises or chirp before any ‘seat of the pants’ indication is noticed. Same with the center bearing, the shaft may vib, but the insulator usually comes apart before and real shaft wobble is detected.
If in neutral, above or close to 35,,, you then raise the throttle, does the vib increase with RPM,, or decrease as the speed falls off (even holding then RPM?)
Is the vib more harmonic in nature, more of a detectable drivability/misfire, or even a thumping low resonance sound or feeling ?

Hayapower, The vibration is most notable between 35 and 40 mph. The vibration decreases as speed increases. The truck makes no notable noises. No chirps, squeaks, or roaring. If I hold the speed at 35 mph the dash, and doors shake and squeak. I can put it in neutral and the vibration will remain until the speed falls below 30 mph. So vibration will start at 30 and progress and peak between 35 and 40 mph then it will begin to taper off and by the time it reaches 55 or 60 the vibration has almost gone away. The tires are new, freshly balanced yesterday. That center carrier bearing is original, but has no visible signs of failure. I'm thinking U-Joint, Center carrier bearing, or maybe front end alignment? The rear end differential at the U-Joint is dry with no oil leakage.
The only codes I have gotten in the past was the code indicating a failed charcoal canister vent solenoid which I replaced and the code never returned.
I also got a code indicating the transmission selection indicator module failed which I replaced and the code never returned.
The last code was the P0301 misfire, I replaced the Plugs, Plug wires, both coil packs, air filter, back flowed, tested and cleaned the injectors, I also replaced the crankshaft position sensor. The code went away and has not returned.
The EGR was replaced about 10,000 miles ago along with the throttle body riser was cleaned out. The hole was completely caked shut. The intake upper and lower are new along with all the knock sensors.
I did not replace the tube that connects the EGR valve to the exhaust. Do you think that tube could be clogged? The Catalytic converters have about 10,000 miles on them as well.

Update: I jacked up the rear end and put it on jack stands and ran the truck and it still vibrated starting at 35 MPH so I removed the rear tires and ran it up to 35 and it still vibrated. I removed both haves of the driveshaft and ran it again to 35 but this time No vibration. Question: Without a driveshaft should the transmission be turning while in Park? Mine does.... In fact to stop it I had to put it in neutral and kill the engine... Anyway, I replaced the U-Joints and the Center Carrier Bearing and reinstalled the first half of the driveshaft assembly, started it in neutral then slipped it into drive and ran it up to 55 MPH with no vibration. I then changed out the U-Joints on the second half of the driveshaft and reconnected it to the differential, but then I decided to change the rear end oil. While the cover was off I went ahead and pulled the Axles and changed out the outer axle bearings and seals. I was about to close up the pumpkin when I noticed inside the differential that the two side axle gears and the two planet or spider gears along with the pin had a lot of wear and slopped around. The rear end is an 8.8 open carrier with a 3.55 gear ratio. So now I'm wondering if the worn gears could have been contributing to the vibration? The rear end wasn't making any noises nor was any of the seals leaking. The ring and pinion are tight with just a little backlash which I think it's supposed to have? So here is where I'm at now
1.) The U-Joints and Center carrier bearing could have been the issue and I could put it back together and it will be fine.
2.) I don't know if I can rule out transmission or torque converter problems. Does anyone know if the transmission or torque converter would cause the truck to vibrate if the transmission is not hooked to the
driveshaft?
3.) I ordered the gears and pin from the Ford Dealership and they will be here in a week. I'll replace and refill with Motorcraft synthetic 75W140 and run it back up 55 MPH and see where I'm at then.
I'll update again when I complete this next phase.