You were lucky. Next time you know something is obviously wrong you might wanna consider checking it out before it goes "kaboom". It would've been a worse event if it'd happened on the front and in dense freeway traffic. Thunky / bumpy type things that change frequency with speed are almost always tire probs. Could be tread separation, flat or belt shifting (tire goes out of round). I've experienced all of the above at some point or other. Only once had it happen with very little warning (right front went kabloom at about 75 mph, < 10 sec of thunky noise before it exploded - tread separation). Most times you can notice something wrong, check it out & put spare on before anything really exciting happens.
Brent
