Mystery popping sound from rear end at interstate speeds?!?!

On a recent road trip a new problem popped up. We'd been moving along at about 80mph for a good hour or so when a dull popping sound starting coming from what sounded like the rear end. It was not accompanied by any vibration or loss of power. I pulled over and noticed a smell but could not identify it. Not tires, or oil or brake smell as far as I know. I had a shop look it over to make sure nothing was loose or rubbing and all was good. They were unable to diagnose the noise though. They couldn't reproduce it on the small roads around the shop. So I carried-on on my way keeping the speed down in the 55-60mph range and there was no further problems. Soo, there is a problem but it only seems to be an issue when the engine has a load for an extended period of time, ie. 3000rpms in 5th gear in FL summertime heat. I suspect maybe what I was hearing was a very intermittent engine misfire? No check engine light. The truck has 241k miles and probably hasn't had a plug change in 140k miles. Just throwing that out there. Where should I start looking?