All shock absorbers use oil as the dampening material.
Some in addition to JUST oil add inert gas to aid in anti-foaming, or even in high-pressure valve control.
Our factory shocks were high pressure gas/oil units from Bilstein.
High pressure just means that the shock has more gas pressure internally than low pressure units, and the gas plays a larger roll in controlling the valving.
Even "air shocks" (the sort that muscle car guys used to jack up their cars) use oil in the shock. The extra air just inflates a bladder (like air bags) to lift up the shock.
Replacement shocks can be just oil, low pressure gas/oil or high pressure gas/oil. Additionally, they can be double tube (have a tube inside a tube (the standard garden variety shock) or monotube (one tube sealed on each end -- the high performance variety). Valving can change, as can shock diameter, shock stem diameter, mounting system, and external adjustments to valving. The larger in diameter you go, the more hydraulic force can be applied to the piston that rides up and down inside the shock. This typically gives greater shock control -- until you get too much -- when the shock becomes a pogo stick instead of a shock. Size matters.
Two great replacement shocks for the TrailVoy platform are the Monroe Sensa or the Bilstein HD. Both are available at shockwharehouse.com, a vendor on the board.
Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum
A forum community dedicated to Chevy Trailblazer and GMC Envoy owners and enthusiasts. Come join the discussion about performance, modifications, troubleshooting, towing capacity, maintenance, and more!