a Boeing 757, registration XA-RLM, was flight planned out of Gander enroute to Acapulco. While the aircraft was taxiing for departure RWY 13 via taxiway Bravo, the tower was advised by a snowplow operator that they had found what looked like a piece of wheel assembly on the same taxiway. The B757 was advised and the crew returned the aircraft to the ramp. The aircraft was inspected and the tower was advised by the Duty Manager that the aircraft had a broken strut. TSB evaluating. UPDATE: The incident occurred after landing. While taxiing to the ramp, ground personnel noticed that the righthand outboard aft wheel was wobbling. Closer inspection revealed that the axle was broken. The failed axle will be sent to Boeing for analysis. Registration was XRRLM instead of XA-RLM.