Suspension thumps when starting from a stop

I have a 96 Bronco, 5.8, 4X4, with 135,000 miles on it. When accelerating from a full stop, there is an initial single thump that accompanies the body's slight tilt to the rear, even when smoothly accelerating. When I last had the tires rotated the mechanic tightened a loose bolt that he said restricts the range of steering. This solved the problem for a while. I'm sure checking the bolt again could temporarily fix the problem, but though this might be a symptom of a bigger problem. Has anyone else experienced this?
Thanks