Hey justinwebb From my experience as a mechanic for some time know I would have to guess that your Catalytic converter has broken up inside and clogged up the flow of exhaust. sometimes it's easy to hook up a pressure guage to the O2 bung and see a pressure of over 3-5 psi but a real quick easy test is to crawl under her and loosen the bolt that hold the downpipe to the rear manifold so that you have a gap and tha en go for a quick drive it should be back to normal (a bit loud ) but otherwise it will accelerate as it did before. also you may want to stick a fuel pressure guage on her to see whether you have the 35-45 psi under a load while driving, but I'm guessing the cat's screwed (done lot's in IL) because it sounds like it happened suddenly then got worse (as it clogged up more and more!) also if you rap on the cat housing with a rubber mallet you will probably hear something rattlin inside. with monolithic cats this stuff can actually get back far enough to clog the muffler too.

hope I could help you with this suggestion (very little investment in loosening two bolts to se if the cat's the prob!)
-Schlimthecarguy@comcast.net-

wheel bearings tend to make a grinding sound- hate to say it but.... your squeak is probably your brake pad sqeakers! when they are just starting the sqeaking will go away when you hit the brake because the pressure dampens the sound (like sound dampner material for a stereo!) if you pull them apart i'm betting that one of your pads is worn enough to do this-check your caliper slides if they are stiff you will need to remove the rubber and grind away the rust that accumulates and causes the rubber to "crimp down on" the slide bolts. hope this helps with that problem.
Now for your tensioner issue you will need to get a belt tesion guage and check it against factory specs but if you look at my prev post-"grasp the tensioner and pull the arm in and out perpindicular to the arms tensioning motion-if it moves a fair bit then the tensioner is posibly rubbin metal on metal (the plastic bushing has worn away)"
-the harmonic DAMPER (it doesn't ballance anything-it dampens (cancels) the crankshaft twisting harmonic vibrations produced during each cylinders firing-but i digress this is getting into the physics if an Otto cycle engine) is actually a common problem with these engines-you may be better off getting an aftermarket one. your alternator is probably due to a stereo/lights and accesseries that it has to power-a bigger (higher amp rated-even a stinger/mean green=>aftermarket) one would help reduce the load the belt has to drive.
Check the tensioner out and maybe replace it and get a good belt (Goodyear-etc) and see if this helps. I have had issues with "cheap" aftermarket water pumps (dont know enough of your vehicle to really comment) and my shop now exclusivly installs AC Delco ones (deallers have them as well as some aftermarket parts stores-check around!)
Hope that i could help you some but I dont like getting into too much detail without being able to stick my eyeball on it-and see how it is.
have fun, Schlim----> Schlimthecarguy@comcast.net