09' 9200 Day Cab 280" WB Steering Vibration

I purchased a 09' 9200 day cab 280" WB that has a vibration between 53-57 mph. It causes the steering wheel to oscillate and the truck to shake back & forth. We haven't even attempted to hook up a trailer yet. It happens when you are just bobtail. The truck came stock with a 10 speed. We had the dealership swap that out for an 18 speed (2x first trans overheated). We also had them install a locker in the first drive axle. The dealership where it was purchased has done everything possible to try and correct the problem. Looking to see if anyone has a solution or suggestion to fix it. Here is what we have done to try and correct the problem.

-Alignment (3x)
-New Tires (Steer & Drives)
-New King Pin & Load Bearings
-Check Wheel Bearings
-New Front Drums
-Install steering stabilizer kit on front end
-Adjust Ride Height
-Check Frame Alignment
-Check Motor Mounts

-We are pulling all 4 drive lines in the AM and sending them out to be checked for balance and any defective u-joints.

The dealership has 2 other trucks on the lot. We drove both of those and they have same issue as what I started with. Of course when I installed new tires on the steer it eliminated 80% of the problem so flat spots on the tires isn't what is causing it. These 2 trucks have the stock 10 speed with no additional work done to them. They all came from the same fleet and sister trucks to the one I bought (1 vin # off)

Only remaining things is to check for a bent axle in the drive or these drivelines have a bad u-joint or are unbalanced. It is just hard to imagine my truck plus these other 2 trucks have bent axles or unbalanced drivelines.

Any suggestions or feedback would be welcomed.