Unless you race your Envoy or have reeeeeeaaaaaaally cheap rotors....it's probably not warping. Rotors need to get extremely hot to warp. More than likely it's rotor slip. Look at your rotors, if it looks like there are 'smear' marks around them, that's causing the vibration. Crap metal, crap pads and just plain crap gets deposited on the rotor surface and allows the pad to "slip" over the deposit, then the pad hits a clean section and grabs normally. Speed this process up to driving speed and the slipping/grabbing turns into a vibration.
Turning or replacing rotors is the answer. Happened to my TB, my previous saturn with 105k miles and 3 sets of rotors and happening now to my Nissan.
Assuming you don't have a runout gauge. May also be a slightly warped rotor combined with loose suspension parts that make it vibrate worse. Why restrict your thinking to only one rotor, though? If one's warped, you'll probably be replacing both, or trying to turn both. Brake jobs aren't a place I recommend cheaping out on. You never replace just one side's pads at a time. I hope.
