What is the difference in the 1500 & 2500 frame beside the frame thickness, and what is the difference in the 02-05 1500 & the 06-08 1500 frame?
Why cant you put a 02-05 4x2 suspension lift on a 06 4x2?

 

The front suspension was redesigned in 06 which is why the kits won't interchange.
The 1500 uses a rack and pinion steering system while a 2500 uses the old style gearbox and drag link set-up so that part of the frame is going to be different.