vibration (not death wobble)

[macofish]

2000 Jeep Cherokee Sport 4WD 4.0L. I've been having this problem recently where upon starting the car I'm getting very bad vibrations. The best analogy I can make is it feels like driving over the lane guides on the highway, (the ones cut into the road). It happens after it's started and usually when idol; like at a bank or fastfood drivethru. It has happened once or twice going fast but is more frequent in idol situations. The only thing I did associate when it was happening is that when it's cold out it doesn't seem to happen at all; only when it's moderately warm to hot weather.

Thanks in advance.

Marc