Torque does play a factor in spring sag and frame twisting. Having been in the auto frame trade for 25yrs ,I have straightened every type of frame from unitized to large truck frames ie. KW,Peterbuilt,etc. On a h/d truck the engine torque to the right doesnt do much,its the rebound to the left that does the damage. I suggest you take the truck to a high quality frame/alignment shop in your area. They should be able to solve the problem. Frame twist and spring sag are identical in nature. If the truck has never been in an accident,has been used in heavy hauling,or has over 100,000 miles it may be worn out springs or spring bushings in the rear but then again with use,the frame rivets and bolts can loosen causing the frame to lose its factory settings. Go to Carstar on the net and they should be able to locate a reputable shop in your area. Keep us informed on what you find.
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1999 2500 Cummins w/4spd auto,Quad Cab,SWB,4x4,Flame red ,all options less leather and radio controls on steering wheel,OEM color matched runningboards,DeeZee bedrails painted black,Power Vision Mirrors,OEM ventshades,Oem sill covers,OEM underrail bedliner