Trying to help a good friend of mine with his truck. Its a 2003 F150 4x4. The truck has a vibration that is very noticeable between 40 and 60 mph. The vibration isn't consistent in occurrence or frequency. Its hard to even tell where its coming from because its not a super strong vibration but its enough to notice. He has had it to two mechanics both of which said they couldn't feel it. This has been going on for at least 4 years now with his truck. At the time it started the truck was his daily driver but its only a backup now. Truck only has 120,000 miles on it. He put new tires on it thinking they were the issue. Didn't help at all. Had the wheels re balanced thinking they were out but that wasn't it. Everything in the front end is tight. No play in the steering or wheel bearings or ball joints. We replaced all the shocks thinking maybe one was bad but that made no difference. Replaced the universal joints in the driveshaft thinking one was bad and still no difference. I'm at a loss as to what it could be.