When driving, vehicle feels like it is lightly bucking

(at low to med. Speeds) / like going over shallow grooves in road at high speeds. The light buck (low speeds) and the groove/rumble feeling (high speeds) only lasts for about 1 to 2 seconds at a time. Does not happen all the time, just occasionally. Seems to do more if vehicle is loaded or if going up hills. I am thinking this may possibly be a Torque Coverter issue or possibly even a rear end issue? The vehicle shifts very smoothly and makes no sounds or noises. Any insight as to what this might be?