Grab yourself 4 car stands, set each to same height and jack up the truck, placing the jack stands under the chassis rails not under the suspension components.
Now measure the distance from ground to cab lower edge front of each door and rear of each door...making sure you pick the same point on each side to measure against.
By placing the car stands under the chassis you eliminate suspension failures
If you get different measurements side to side, it is usually the cab mount sections failing...sometimes its the rubber components, but usually its the cab mounts rusting away and collapsing...
Spacing them up with washers is a bad idea as the cab mount will only continue to fail...replacement cab mounts are available...
BTW...it is usually a good idea the replace all cab mounts at same time...
Have fun...
