I was just reading through some of the tech pages on another thread. From what I can tell, if you can get the truck up on pretty level ground, your looking at about 27"-28" to the top of the Bumper. Or from the top of the cab to the ground, 73" this is figuring you have stock tires and suspension. It's probably not perfect, But a pretty good starting point.
Like Chris said, start checking the Cab mounts, Maybe a collapsed shock. Get yourself a floor jack and a 4x4, jack the cab up on the low side until it levels out and check the difference in all the Body mounts. JMO
Rob..