My uncle has a 2002 Ford F-150 SVT, he is facing some vibrations on speeds 50 to 90 MPH.

He thinks its from the drive shaft but he want to make sure before ordering it.

I'm sue you guys know the problem and can shed some lights

 

Have the wheels been road force balanced? Any loose front end parts?

 

50 to 90mph? Your uncle's a speeder, eh?
A couple of places on I15 in Utah post 80mph test stretches and some freeways in Texas may still have 80mph speed limits & that's plenty fast in a truck.

 

No loose parts and wheels are balanced

 

We live in Dubai, UAE. The maximum legal speed limit can reach up to 100MPH. You can do more on open roads where there are no radars.

 

could have a bad tire, my 4x4 vibrated around 50+, and when i got new tires it went away.

 

Ah, well forget about our piddly low speed limits then.
Montana had, for some time, a basically unlimited speed limit. It was "safe and prudent" so sometimes that was at the discretion of the state troopers.
That limit dropped to 55mph at dusk and was, from my experience, strictly enforced.

As to the vibration... I would also look more at tires or other related rotating parts than the drive line. There's certainly a possibility of some type of non balance tire issue going on. That said, something in the drive line could be the problem as could engine misfire.
How and where is the vibration felt? Steering wheel? Seat? Feet? Can it be detected more on one side or the other?

 

Take a look at the u-joints

 

If its the drive shaft it may just need to be rebalanced. I had a weight fall off of mine and it caused a vibe between 50-60 mph.

 

Probably the rear end modifier and fluid is spent. That's what they do @ about 50 mph, -shake and vibrate like crazy when that happens.

 

Tires are okay and the vibration felt at feet area

 

Have you checked the engine and trans mounts and checked exhaust mounts and the exhaust for component interference?

 

I will let him check, so its nothing to do with the drive shaft?

 

I believe a drive shaft vibration is going to be felt in the seat more than the feet. Most times I've seen drive shaft vibrations felt in the seat or through the entire body/cab where you can kind of hear it resonating.
It certainly won't hurt to have the drive shaft checked for balance since it should be reasonably inexpensive and then easily exclude or confirm that component as the culprit.
As to the mounts, have them look for loose connectors, collapsed or oil soaked rubber cushions or even the rubber cushion separated from the metal mount and correct alignment of the mount assembly. (there may be wear marks or marks from the original position if the mount shifted)
Look for pretty much the same for the exhaust hangers/insulators and even one missing hanger/insulator could cause the kind of vibration that could be felt through the floor.
If, by feet, you mean gas pedal, you may have to look elsewhere but it could still be engine or trans mounts or exhaust related.

 

I'm also guessing tires, like a mild version of the "death shake" you can get in a big 4x4. Even with each tire balanced one could be slightly out of shape compared to the others and cause a vibration. Happened to me in my old truck from about 40-55mph it would shake like crazy even with each wheel balanced. When I got new tires it cleared up the problem. I'd see if he can get a different set of wheels and tires from a friend just to try out to make sure.