Rear Suspension problems of the 1998 GMC Sierra

The frame resonant vibration frequency on this truck precisely matches the tire rotation freauency at about 45 mph. This causes very anoying bouncing of the rear end at 45 mph. When brakin at 45 mph, the rear end jumps about uncontrolably and could easily lead to an accident. I believe it is a design flaw. Only the extended cab models have this problem. If one uses the exact same frame that is on the regular cab model for the extended cab model and simply makes it longer, the stiffness will be less and could lead to this problem. I believe it is the result of inadequate frame stiffness, and I believe GMC knows (and has known) this and has made a business decision to put up with the complaints rather than fix the problem. All owners of similar models of GMC and chevy that I have talked to have told me they have the same problem. Gm should not be allowed to ignore a potential safety problem.

Within 10,000 miles the rear end of the vehicle has been replaced three times. Cause unknown. Please give any further details.