Jeff,
If this is a periodic motion that occurs when driving on a roadway at about full speed (probably around 10 mph), it is called road lope. This is due to something slightly out-of-round in a tire / wheel / hub assembly. The out-of-roundness causes a once per revolution pulsation to the tractor at or near one of its natural frequencies of vibration (bounce or pitch).
You can watch the front tires as you drive to see if anything is noticable there, but you can't really see the rear tires from the seat.
The simplest solution is to just slow down a little. It will usually go away.
JackIL