Truck shakes at low RPMs?

[lbrowning53]

So i noticed today that at about 1500 RPMs, my truck would sort of.. vibrate I guess. It only happens at 1500 and its just a really low hum/vibrate. It has never done this until this morning. The only thing I've done since then was raise my exhaust up about an inch.. I have a 3 inch body lift, so the stock height of the exhaust bothered me. So i took the very last hook of the stock exhaust holders and wired it to the frame. It seems sturdy, but this shaking didn't occur till I did this. Any theories?