Started to hear (and feel) a low pitched rumble in the drive train. When it became loud I would take my foot off the gas and shift to Neutral which typically caused the rumble to get louder. But then putting it back in gear would make the rumble go away - for a little while. Took it to my local mechanic who dropped the rear drive shaft and found both U joints were bad. $250 later, problem seems to be resolved. I was expecting much worse financial damage.