So the wife and I are on are first trip w the new truck w/o the trailer. We find our self with an intermittent vibration coming thru the lower seat back. While this usually suggests rear tires that need to be balance, I don't believe this is the case. i stopped ina town and had a tire dealer check the balance of the tires and all was ok. What is odd to me is that this happens on smooth roads with the cruise on. it will vibrate and shakes pretty good, then stops. It doesn't doesn't seemto be a tire thing, and the speed and load doesn't seem to be changing. If it was tires I would think it would get even worse at higher speeds, but we don't notice it. Its almost like an on / off rumble.
I have the EXACT same symptoms!
I bought a 2012 F250 with 20" rims 2 weeks ago from Greenway Ford in Orlando. I live 150 miles south of the dealer in Stuart, Florida. During my initial test drive, I encountered the same scenario you described above, but it started at 65 mph. The service manager assured me the tires just needed balancing, so I signed the contract and before taking delivery of the truck, the wheels were balanced. We took a test drive at 65 mph and all was well, so I headed home.
Once I hit the turnpike and got up to a cruising speed of 75 mph, the vibration / shaking started again. Like you, it was most noticeable on a smooth road at a constant speed. The shaking faded in & out, with a few seconds of smooth ride, followed by a minute or two of shaking.
When I returned home, I took the truck to a local tire shop that preps all of my race cars. The tires were road forced and it did not fix the problem. It is worth noting the Michelins needed upwards of 9 ounces of weight to balance and two tires needed to be reset multiple times on the rims for them to pass road force. The ride was definiately smoother after this, but it did not fix the problem.
I took the truck back to Orlando and the dealer replaced all 4 tires, then re-balanced them. We took a test drive and the problem persisted. Next, we traded my rims and tires for an identical set on another new truck. Took a test drive and the vibration was worse, so they re-balanced the new set and it was a little better. Still, the problem was not fixed.
Out of curiosity, I asked if we could test drive another F250 on the lot. When we got up above 70 mph, the shaking began with this truck as well. At this point, we've ruled out rims and tires. There is something else wrong in the engineering of this truck, whether it be the drive train or suspension.
I have read on other forums that changing the drive shaft and U-joints had no effect. I'm thinking about trying new Bilstein shocks and softer shackles. I have not tried towing yet, nor have I put any weight in the bed. I'm curious if this would help.
I have less than 1000 miles on this truck and I'm very disappointed. This is my 3rd F250. Previously owned a 2000 7.3 and 2006 6.0. Both of these trucks were smooth at any speed. I use my trucks to go back & forth from Florida to the Carolinas. The vibration at highway cruising speeds will drive you nuts!