Shaking and Vibration of Ford F250 Turbo Diesel

Help! Our 2006 Ford F250 Turbo Diesel has a problem that can’t be fixed. It only has 60,000 miles on it and did not give us any trouble for the first 57,000 miles while regularly pulling a bumper pull horse trailer for the first 45,000 miles and a two horse gooseneck trailer ever since. It has been at the shop numerous times since 57,000 miles and we still have a problem that continues to get worse. It used to be just once it was warmed up and towing, but now it is happening when not in tow. The problem is the whole truck vibrates when the RPMs are 1750 to 2250 (mostly when it goes into overdrive). It is getting worse and worse. Local Ford dealership has experienced the vibration, but cannot find anything wrong, and says to ignore it until we get a check engine light. It is getting harder and harder to ignore as it is so bad when it happens our voices vibrate when we are talking! Also, our worries of the safety concerns of our 19 yr old daughter driving horses around the southeast are very disconcerting! Any solutions as we await the check light?

First off, find another shop, preferably an independent one.

This may be due to an unbalanced driveshaft, worn U-joint or something else in the drivetrain.

Does the engine run very rough, warm or cold?

It has been to 2 separate ford dealerships. Nothing shows up on their diagnostics computer. Supposedly, it is still under warranty for the drivetrain. It runs fine until warmed up and then starts with the off and on shaking, especially when going up a long hill, although sometimes just on the flat.

Update: They checked the driveshaft and the u-joints and said they are fine. Any other ideas?