Thanks, everyone, for your responses. I'm forwarding them to my husband since he knows way more about this stuff than I do. I've had the Disco since 2007 and have taken a ton of road trips in it (including on backcountry roads), so I have a pretty good feel for driving it; I just don't know much about how the insides work. The flex disc was replaced in 2011, so while I barely noticed this "thunk" that seemed to have started then, it is now (four years later) very noticeable. I'll have the items you've all mentioned looked into.

 

Check the differentials if it seems like its getting worse. I had a customers car have the bolts in the carriers back out, the front diff exploded and punched a hole in the front of the axle tube.