I guess this is a bit old now, but I run 6* of caster in mine. And I never let the alignment shop do my toe in because they all F it up. They always set it to factory spec which is a 28" tire but I'm on37"s so the toe in needs to be adjusted.
First I'd align the front for the tire size you have. After than I'd try different tires. I had this same problem with Noahrod after he linked his truck. His what the steel bead locks and bias Irocks. The bead locks aren't balanceable and bias Irocks get flat spots in them. And he felt the shaking between 45-55 too.
If want you have is the same as Noahrod's problem then its not death wobble, its just the tires and/or wheels that wont balance. Death wobble typically only stops when you stop the vehicle. A good way to tell this is, toss on a different set of tires and wheels. If the problem persists then you know your tires and wheels aren't the problem.
