I have a 2003 Century C12 18SPD stick and 20/145 diffs. Never taken any notice (until I had to check and tighten the U-bolts) the small height difference in the chassis. Drivers side is 20mm higher between bump stops but the front tow hooks are the same height. Obviously the drivers side air bags are inflated more but the airbag base on the 3rd axle is 15mm lower than the other side. Basically to just say what I'm thinking...it seems like the chassis has a slow bend starting somewhere under the cab and the rear diff has a twist. There is something wrong ....just don't know what.