What's the "thud" I'm feeling underneath sometimes when coming to a stop?

What is your question?

I feel this single "thud" or "clunk" under middle of the chassis when coming to a stop at a light, but only occasionally. This won't happen right away but maybe a second or two *after* coming to a full inertial stop. When it happens, it's not subtle. I have the 4WD SR5. I've felt similar noises in other older (high mileage) 4WD/AWD vehicles but never got a precise diagnosis for those either. Any ideas? Is it serious?