Heavy Idling '95 bird

Alright, I've gotta '95 firebird 3.8L V6, and i got some questions.. the car is slightly old.. about to be pushing 130thou, so ive been expecting some problems from the car.. It started with some rough idling, so i went and got the car checked out, the guy said it was a missfire on plug six, and my egr valve was malfunctioning... and he said the egr could be as a result of the 6 plugs.. so i jumped to it and changed all my plugs.. (#6 being the hardest to get to :-P) No change at all, in fact the car got worse as time went on.. people have suggested a bad egr, bad plug wires.. since now the idling is insanely rough... i dont enjoy the shakey ride :P Also ive had problems with my heating.. like if i have my heater on too long, it will start blowing cold air and my car will begin to over heat.. im thinking thermostat but im not sure.., i also have a slight water leak around what i would like to believe is my water-pump in my engine... (the wheel on the belt below the erg valve, correct me if im wrong) and sorry for the extensive-ness of this post.. but one more question.. i get a wierd whining noise whenever i accelerate/ slow down, coming from the rear.. possibally low differential fluids?, (oh yea.. my acceleration is very flakey as well as the idling.. very slow acceleration) thanks if you guys can help me out any :D sorry for the length.. peace.

Alrighty, i changed out the plug wires, turns out, wire #6, was burnt out on the inside and wouldnt fasten to the contacts, and wire #5 was physically burnt and cracked, My idling has calmed down a TON, still a lil bit shakey but not half as bad as before.. much better, And accelleration has also improved, and it no longer back fires... stilll questioning that slight idle problem i still have.. and still questioning the differential question thankees :D

have you had all the codes that were set cleared? if not then the car is trying to "relearn" where it should idle with all 6 working again. Unhook the battery for a few minutes and hook it back up, this should clear the codes, good luck.