My heat shield over my muffler came loose and I had a mechanic tear it off until I can get a new one so it's not scraping on the pavement.

I agree with Wally Jay... However you stated.. "so it's not scrapping on the pavement ".. IF it's the *lower muffler* heat shield, it should be ok, as long as there are no exhaust leaks that could possibly let co gases enter the cabin!!

As long as your vehicle is moving along at highway speed, the heat will be dissipated, and pose no problem. However, the shield is there for a reason, and should be replaced at your earliest opportunity. In particular, if the heat shield is the one on the catalytic converter, I personally would not take the trip with that shield missing. Catalytic converters can get much hotter than the rest of the exhaust system, and I would consider that shield a critical safety item.