How much of a job is it to locate & replace thermostat?

Its not too bad, onthe 3.8L it is located on the left side of the engine on he top near the intake plantum, and I think the 3.3 and 3.8 are the same block. The haynes manual will guide you through.

Is he vehicle overheaing?

No it wasn't overheating, it just would not blow any hot air. I did replace t-stat, it took all of 20 min. I wish all repairs were that easy. Thanks for the input.

Our van will warm up to correct temp (and blow hot air from heater)when idling, but once we start moving, the temp gauge goes back to C and the heater blows cold. Would this be the thermostat?

Our van will warm up to correct temp (and blow hot air from heater)when idling, but once we start moving, the temp gauge goes back to C and the heater blows cold. Would this be the thermostat?

It could be, I'm by no means an expert but it sounds like a t-stat problem. One thing about it it's a very easy process to change out. Drain water to below top hose, remove the 2 bolts holding t-stat housing cover, (1/2 bolts located upper left side of engine, follow upper radiator hose), replace t-stat, remember the direction old one was placed, reinstall housing and re-fill fluid. Let run for a while with cap off to bleed any air out of system, squeeze upper hose to "burp" a couple of times. When no more bubble show up system should be bled. Hope this helps.