baskervi

I have a 2003 F150 7700 with 4WD that started to sound as if it has mud tires on it when going down the highway - but it has highway tires. That happened a couple years ago just before the tread came off of a tire, but I stopped by a tire shop and everything looks OK. I also noticed that when I was going around 40 and jumped on the gas, I could hear a slight popping sound. If you could imagine a couple of gears that are too far apart and they are slipping, that's the best definition I could give it. It drives straight, but when it's grumbling, I can feel very slight vibration on the steering wheel. I can't pinpoint exactly where it's coming from, and I'm not exactly sure where to start checking this out at. Does anyone have an idea? Thanks