You don't need a vacuum gauge, but an exhaust back pressure tester. It'll screw into the oxygen sensor bung (you have to remove the sensor, obviously) and do the test with the vehicle cold. Since you have a V6, you would have to do both sides to do a proper test. You'll want to install it in the upstream oxygen sensor bungs, which are best access through the wheel wells. An exhaust back pressure tester looks like this:
http://www.oreillyauto.com/site/c/detai ... /N0715.oap