I do not see where your Envoy is equipped with "rear" air bags. I see front air bags in the dash and side air bags in the front seats. In order to replace the side air bags the front seats must be removed and partially disassembled.
Get a pair, with a lifetime warrantee, for $200 total, from Arnott. Just need to jack up rear and use jack stands. No other tools needed. Instructions come with the bladders and take about a half page with big letters and diagrams. Very cheap and easy fix to a problem that seems to vex or break the bank of so many others.