Oldcalcutta has the high probability answer - especially if you off road in rocky areas. It could also be a bent wheel, a thrown wheel weight (I use to have this happen all the time on my Cherokee with alloy wheels but never on my Sammi with steel wheels), debris stuck in the wheel (ice, snow, mud, a rock even) (an ice clot scared the heck out of me in the Sammi once), or a bad shock.
I've seen weird problems with trac control that would pulse one wheel so I guess antilock brakes could do the same thing. We found the trac problem when the tire showed wear.