Everytime I go over potholes or just uneven pavement, I get a loud rattle or clunk, like something is loose.

First thing I checked was the RSB but all the bolts are on tight.
Next thing I thought were the struts (50k miles on a 2006), but the dealership blamed the RSB.

 

Almost definitely your rear sway bar endlinks. They attach to the very ends of the RSB -- take a firm hold of it and wiggle it -- if there is ANY play, that will translate into a clunk when driving.

What happens is the ball joint at either end of the endlink can sometimes fatigue or fail, causing too much play between the ball and the socket.

Replacement is simple - 5mm allen key and a 14 or 16mm wrench. Allen key holds the post from spinning while you turn the wrench to loosen the nut. Installation is the reverse, and from my experiences on numerous cars there is no difference between bottom and top joints.

 

Ya, it's a pretty easy R&R, but they might just be loose. And you can't really tell be trying to shake it. I was getting noise from the OE end links and although I couldn't moved them by hand, I tightened them up and the noise went away. If you do replace them, get the Moog aftermarket end links. They did away with the PIA stud you have to hold with a allen wrench on OE ones (you use two wrenches), they're allot beefier, and they have grease fittings top and bottom.

 

Even if its just slightly loose, it is detectable (at least by me?) by shaking it. ANY clunk or feeling of a tap when doing this means something has play, and play isn't good in an endlink.

Maybe its the sensitivity I have in my fingers as a result of being an avid fisherman... relying on contact with the rod blank to detect a subtle bite?

That being said, i've never had a loose endlink -- all mine have been snug and its a very very minor bit of play in the balljoint itself (probably due to grease escaping and the balljoint corroding and turning into rust-dust inside of the boot!). I have had Moog's with and without the double-nut fastener method, even ones with the same part number! Talk about frustrating when you order a pair for your wife's car and one comes with the allen key method and the other comes with the far superior double-nut and zerk fittings!

 

Interesting that Moog made them with the fasteners that required the allen wrench. I'll asume the double nut style in a newer model. And I'm not surprised that you haven't encountered loose end links. I didn't want to go into the long story about why one of mine was loose (and I won't), but I agree that that's a long shot.

 

Ughhh... I stripped the hex screw.
How can I get it off and where can I get a replacement?

 

Use your best friend -- the angle grinder. Cut through the nut and the stud at the same time, just being careful not to cut your hand or the flange that the endlink attaches to (or the swaybar if its that side that's stripped).

I've done this probably nearing a dozen times on my wifes' car and sisters' car (both Protege5's that love to rust, ARGH!).

 

And a replacement can be bought at any auto parts store or beefier ones from heeltoeauto.com. It IS ok to drive without the sway bar on or the endlink(s) attached... just expect that the car will have the rigidity of a noodle and will understeer pretty significantly. Don't drive if not necessary, but it is ok to drive in a pinch!