Odd shimmy

[JohndeereG]

When I leave off the gas in either 1st or 2nd gear, I get this shimmy in the seat of the truck. I don't notice it very much in the higher gears. I first noticed it yesterday as I was driving through a flat, paved parking lot when someone started backing out further down the row. I just left off on the pedal to let the engine slow me down but I got this noticable shake, shimmy, vibration (you pick the adjective).