Originally Posted by RatPin
So ruling out the possibility of a drum brake dragging, nobody has an idea of what would be causing this? Could my diff be bad?
That is normal for an open (non-posi) differential.
As to the brakes, as was mentioned, check the pushrod clearance between the booster and master cylinder for a small amount of clearance, and make sure your brake pedal returns all the way so there is no pressure on the master cylinder and booster when released.
If you have your front brake lines attached to a 10psi residual pressure check valve that will make the disc brakes drag to much, but not so much so it won't coast when your foot isn't on the pedal.