just installed a pro comp 6" lift on my 07 crew cab chevy 1500 having issues with vibration during highway speeds in 2wd and during breaking all blocks are installed correctly per instructions. it seems to be coming from the rear end not the front the whole truck shakes. any ideas  

First you might get tires and wheels spun balanced and next inspect drive shaft joint angles. Ujoint are not constant velocity when bent and can cause a lot of vibration when run at higher angles. You problem is kinda common at lift of 6 inches and above. Generally it is not seen with 4 inches and less. Also which 6 inches and more you really need to lengthen drive shafts too. One more thing, With 6 inches of lift on your truck and the likely big tires with it you are flirting with fire because your axles are kinda weak front and rear and you are going to have short front suspension life and short tranny life too if you have not regeared axles properly and dealer could choose not to honor tranny warranty with lift is axles are not regeared. For what it is worth....