Q&A

The hole in your muffler is not likely the cause of the driveability concern with your 1991 Chevrolet Blazer. Also, your Blazer is 18 years old, the $400 you have already spent may only be the beginning. The ignition and engine computer systems would be good place to start looking to try and correct your "sputtering issue". Checking the engine computer for fault codes and inspecting the distributor inside and out would be the first steps in trying to diagnose your problem.

Check and clean the EGR valve.
They are known to get clogged with carbon build up. They sell an EGR gasket with a fine screen attached to it. This screen helps keep the carbon out of the EGR valve.
TO REMOVE THE EGR VALVE.......
The EGR valve is behind the tension pully:
Losen the two plastic wing nuts from the air intake assembily and remove the assembily.
Take the tension off the tension pully (5/8" breaker bar or long wrench), remove the belt from the alternator pully then guide the pully down and release.
Take note of the position of you serpentine belt because you'll have to put it back. Leave the belt in place after you take it off the alternator pully.
(Don't foget to unplug the wire to the EGR valve at some point)
Now you can reach the two bolts holding the EGR to the block. You need a 10mm socket with one short extension (for the bolt on your left) and one 6"(approx.) (for the bolt on the right).
As you losen the bolts you will see a support bar attached to the pully area this bar will be in your way when you try to pull the EGR valve out, so take out the top bolt(14mm ? ) and push the bar up and out of the way.
Now you can remove and clean the valve.
Reinstall the valve and gasket.
(Don't forget to plug the wire back in)
You can reuse the gasket if it's in good shape. The smart thing would be to go and buy a gasket before you even start.
This is an easy and cheap way to see if this is your problem. If it's not that then have it diagnosed at a local station. Some of the smaller stations might hook it up to the scanner and check codes with out charging you.
Also check your spark plug wires I have seen them touching the manifold and overheating or even burning through.
Good luck.