Rough running engine

Recently my vehicle started running really rough and it sounds like it has a miss. Seems to sound alright at higher rpm's, but when I let off the gas the rpm's drop very low to around 500. Smells like it is running very rich and is seems to lope. Plugs are all gaped correctly, cap and rotor are only two years old, timing is correct.

Happened out of no where while I was driving to work doing 70 mph down the highway with the cruise control on. The truck downshifted going up a hill rpm's went up to about 2700 and when I hit the top of the hill and the rpm's dropped down to about 1700. It started sounding rough and tried to stall out when I was getting off the off ramp.