how to strengthen 2x4 box frame on square body burb?

My burb's frame flexes real bad when we try to jack it up. Is 2x4 box tubing too small for that size truck? Is there any way to make it more rigid without redoing everything in 2x6?

My dually twists on the stock frame, its such a long wheel base but the only other thing i would recommend is adding more crossmembers where ever you can.

Yeah you want some flex in your frame. Too rigid and stuff will start cracking. You could always do a frame wrap with an extra layer of 3/16" plate or something, maybe a couple extra cross members.