Lately, about the last week, I've been noticing an increasing 'thump' as I drive. I noticed it first, and still, most prominently on deceleration. It's rhytmic, and seems to be proportional to driveline speed. It's a '91 EB 4x4. It's not engine I don't think - once the valve clatter stops, and the startup 'knock', when it warms up, there's no 'other noise' while not under load. During a non congested stretch, I eased the truck into neutral, and it still continued to thump. I got under the truck without ramps & looked around with a light, I didn't see anything loose that I could easily identify, I grabbed & yanked on the drive shafts, and they felt tight. Wheels & tires looked ok & they weren't 'mudded up'. I can't notice the symptom until about 25 mph, it doesn't seem as bad while accelerating, and the brakes almost seem to 'quell' the symptom a little... Any ideas?