I have a 2005 Chevy Venture 3.4L LT with front and rear air. Where is the expansion valve located and how to replace it?

 

I don't know much about rear A/C, but GM usually uses orifice valves these days (actually since before R-12 was banned for R-134A) but the orifice valve is usually to
be found past the condenser on the lower "cool" side near the bottom before it enters the receiver/drier/accumulator. Back in the 1960's with the old R-12 systems,the
expansion valve was a bitch to get to, as it was on the inlet to the evaporator with the temp sensor on the outlet of the evaporator, meaning tearing the dash apart.

If it is an orifice tube, not an expansion valve, it might well be a specific type for your front and back system, so you should be sure to obtain the proper one for your system
as they do come in many different variations.