clunking when I sit down

Hahaha I'm not fat or anything, but when I flop down in the drivers seat (with the car off so I can hear) I hear a clicking (like something's bottoming out) from the drivers side rear end of the vehicle.

I can imagine it having to do with the shocks or something, but I can't tell if it's doing that over bumps because everything already rattles over bumbs or upon applying the brakes (from brake vibrations).

It also happens when I come to a lurching stop. It might be my spare tire.

How often are shocks to be replaced? How do you know when they are bad?

I know this may be hard to believe but it could be your hood hinge bolts. Do a search on Blazer hood hinge rattle (it actually is a clunking sound). I did mine a few days ago and was TOTALLY AMAZED. You need these little cone shaped washers (they say they are spring washers). I had checked all mine front end parts, and shocks etc. ( was totally convinced I needed major front end work) and then finally replaced these little washers ($2 each). Takes ten minutes and is really easy. Need a 7 mm socket to remove the small wiper cowl and a 13 mm socket and wrench to remove retainer bolts.
Try it! Mine feels like a brand new vehicle now! Let everyone know if it it works.