did you lift the bronco? if so the pinion angle could be off causing the trouible. If not be sure to use good U-joints. the cheap auto zone joints won't cut it. also the u-joints are the weakiest link in your driveline.
With the bigger tires your gonna get alot more twist of the axle,especially if your using a lift block,thats gonna eat u-joints on a regular basis.Like what was said in the previous post if ya did lift it you driveline may be outta line also.