Since you said you have the frame level on jack-stands..try removing "both" rear cab mounts and see what happens. A level across the rear window opening, or the rearcab floor brace (interior at floor and back cab wall) should be a good place.
Hopefully you won't find the cab flexed or twisted. Once both mounts are disconnected,take a measurement from under the cab..from the floor to the top of the frame-rail on both sides..same # ??

 

I had the same problem. I switched sides with my (brand new) "dog bone" style mounts to no avail. I did just as CharlieLed did and the new high dollar mounts solved my problem once and for all. Hope this helps, Steve

 

Looking like that my be the way to go. At this point its apparent that something is tweeked (prob the cab), but the real issue is how to makeup for it.

 

Well, I couldn't justify 200 bucks for the adjustable mounts.. so I built some, they work great, all for about 20 bucks.
heres a pic, how they turned out