Is the rear suspension totally stock? Does it do it every time or only when accelerating hard? Have you gotten under the truck to see if everything is tight and nothing broken? Especially the new parts.
Talked to the guy at JTR jagsthatrun.com. They informed me that it because of the pinion angle. They have a kit for ext. cab 2wd that has a shim block for the transmission, center support bearing, and angled shims for the leaf springs (2,3, or 4 degree) depending on your ride height. 2 degree is for stock height. 3 degree is for ZQ8 or stock trucks lowered 1-2 inches. 4 degrees is for stock trucks lowered 3-4 inches.
They also have an "axle set back kit" that centers your rear axle and gives you about a half inch more travel on your driveshaft for trucks lowered more than 3 inches. (may have to have driveshaft shortened if truck is lowered more than 4 inches)
This is just me passing along what information i got this morning