I do not know if this will help but knowledge is never a bad thing. There is a GM Campaign #PIT3009J. This is a suspension vibration or frame beaming problem that manifests itself at 40-50 mph. Basically if you reference the bulletin number they will replace the rear cab mounts on crew and extended cab models. This is warranty work and does not cost anything. They have ordered the parts for my 07 classic & I have heard that it helps.
Last edited by conway; 11-26-2008 at 18:51.
Reason: addition of info
1) '03 GMC 2500HD Dmax/Ali Ext/Short 4x4; 4" exhaust, egt and boost gauges; exhaust brake; Bilsteins; BD exhaust manifold; Oil Guard bypass; Lubrication pre oem fuel filter; Tekonsha Prodigy; Mag-Hytec differential cover; Suncoast V transmission w/deep oil sump
2) '07 Classic GMC 2500HD Dmax/Ali Ext/Short 4x4'; Bilsteins, Silastic shackles, BD exhaust manifold; Oil Guard bypass; Lubrication pre oem fuel filter; Tekonsha Prodigy; Federico sway bars; Dipricol boost and egt gauges; Transgo Jr,, Mag-Hytec differential and transmission pan