power and exhaust problems

I have a 1991 S10 Blazer. The motor is only 2 years old as of May '05, and has ran great up until 3 weeks ago.
My Catalytic Converter broke and I replaced it, at the same time it developed a REALLY BAD shake and loss of power. The coil wire was corroded at both ends, replaced it, helped a little. Replaced the O2 sensor, used code reader and got nothing but 12's indicating the test was functioning normally.
Now my muffler has become damaged, it has begun to melt the undercoating from the underside of the truck, it is discolored and black. I bought a new muffler and installed it last night. Drove to work this morning, 11 miles, and this one too is now turning black already.
The plug wires look good at both ends unlike the coil wire so I have not replaced the wires or plugs although they are 3 years old. I replaced the coil, helped a little. I had a collapsed vacuum line, replaced it.
What could be doing this to my exhaust, and is it dangerous? Why do I have no acceleration and a bad side to side shaking? Any ideas would be appreciated, I really don't want to keep buying parts that are not going to fix this. I don't mind doing the work, but don't know where to look next, there are so many possibilities from what I am reading online.
HELP !

if your plugs are 3 years old, it might be time for a good tune up. you would not believe how a tune up would improve performance and fuel consumption. cap, rotor,
wires, plugs, air and fuel filters, pcv valve, and check timming.
really that is about where you should start and also it doesnt cost much and if it doesnt solve the problem youre only out 40 bucks and it was needed anyway.

if your plugs are 3 years old, it might be time for a good tune up. you would not believe how a tune up would improve performance and fuel consumption. cap, rotor,
wires, plugs, air and fuel filters, pcv valve, and check timming.
really that is about where you should start and also it doesnt cost much and if it doesnt solve the problem youre only out 40 bucks and it was needed anyway.
Thanks for the quick reply!
What about the exhaust issue ? How is it related ? This has all happened at once. Is the muffler still ok to leave on ? I also melted my plastic fuel tank gaurd with the tail pipe. It is all installed right, the distances just changed with the new pieces and it is closer to the gaurd.
Would the converter going out have caused these problems to surface or fauled the plugs?

this is supid ? but needed asked, are you sure the muffler isnt installed backwards? see stupid ? . why did the distance change? yeah you dont want it too close to anything.
everything that your plugs cant ignite goes straight to the exhaust. so if your plugs are getting weak spark you wont be able to ignite all the gases and all of that will cause clogged cat and mufs, creating a much hotter exhaust.

this is supid ? but needed asked, are you sure the muffler isnt installed backwards? see stupid ? . why did the distance change? yeah you dont want it too close to anything.
everything that your plugs cant ignite goes straight to the exhaust. so if your plugs are getting weak spark you wont be able to ignite all the gases and all of that will cause clogged cat and mufs, creating a much hotter exhaust.
Yeah, it's installed right, A towards the front, B to the rear. The change is just a difference in the length on the new muffler I guess. There is a longer flange where the pieces fit together which has extended the tail pipe and caused it to hit the gaurd.
I called the mechanic that installed my engine and explained everything that has happened and what I have done, he says that the distributer needs replaced, that it is not advancing the timing and causing the weak spark and the excess gas to enter the exhaust system. He thinks that will take care of my problem.
I removed plugs 1, 2, and 4 last night, they look brand new. No sign of any wear what so ever, and the gap was perfect so I am going to get a distributer and see what that does.
I'll let you nkow what the result is.
Thanks

You might have fuel entering the exhaust system.

Replace the cap and rotor and plugs and have the timing checked with the timing wire disconnected from the PCM under the glove box.

just give her a full tune up. Will only cost $40 or so and the problem may go away.

Well, thanks for all the postings, I should have done 1 last thing before I took it to a shop.
I replaced the plugs but not the wires in a final effort to repair this. $194.71 later, the spark plug wire on number 4 had a hole burnt in it. New plug wires did the trick after I replaced the entire distributor on the advice of the mechanic that installed the motor for me, on top of the other items I replaced.
A grand total of $425.00 give or take, 6 weeks of problems, 3 weeks of car-pooling with my wife, and a $60.00 set of wires resolved my problem.

I do however have noise, not from the wires, on my Satellite Radio and regular FM tuner ? It's not the normal you would hear from wires, but a steady constant noise that does not increase with acceleration. Any thoughts?

Thanks