Have you checked the turn stops on the lower control arms? On the lower control arms, out near the lower ball joint, there is a welded on "stop". There is a "tab" on the steering knuckle that contacts the stop when the wheels are fully turned. It is to prevent overturning of the wheels in each direction. If the tab is contacting a dry stop and the body moves up or down, they clunk pretty good. They need to be lubed at each oil change., put a gob 'o grease on each one. You can see the tabs in this image:
Dorman Products - 520-142

 

I saw the turn stops when I was under the truck on Wednesday. They were completely dry and looked like they had been rubbing. I'm going to put some grease on them this coming week. I expect to get more troubleshooting done throughout the week as I'll have plenty of time off.

Any suggestions about what would cause the tire scrubbing? I think that's making the clunking worse as the tire hops. I have to go through and check the 4WD system some more because I got some odd results from when I jacked up the right front tire last week.

 

My 96 Blazer makes the same noise.. i have wondered for years what it was.. it has been inspected, aligned, etc several times and i cant find it. I dont see how it could EVER be aligned properly with that kind of movement in the control assemblies. The tires do not wear unevenly.

 

Toby the same thing happened to me a year or more ago. I searched on line and found a post that at first I thought was a hoax. But the post said that Blazers had this problem and gm never acknowledged it or repaired it. But the post went on to explained that when the tire was turned full right that it pushed up the fender into the hood. And it went on to explained that if you were to put a shop towel or two on the fender and across the hood rest ledge on the fender were the hood rest on it. Just not to many rags as it might bend the hood if the rag count was to thick. So with very much doubt that something so little as this could work but guess what it did. So I bought some sticky back tape stick on both sides but separated with a thin layer of plastic and on the first two layers I peeled off the plastic cover so it was sticky on sticky both sides first two layers. Then on the last layer I left the plastic cover on the top layer so it wouldn't stick to the hood rest but it would stick to the layer before last. Trust me It sounds crazy but it works and maybe if you was to google the problem you might find the original post that resolved it for me.2003 Blazer 4 x 4 S10 4.3 Liter

 

You were a just few days early for this thread's 4th birthday! Good story too... Tire pushing the fender into the hood...

As far as the hood hinge issue you speak of, GM definitely acknowledged it and issued a TSB with the correction. It was documented in the Tech Article - Hood Hinge Rattle - TSB 01-08-63-001B as well as many other threads over the years. I personally went through this on my old Bravada and helped many others through it as well.