On the vortec 4.3L it is easier to access the egr by removng the air intake. Disconnect battery. Having a cold engine really helps too. On the intake manifold under where the air inlet is, is your egr. You need a few extensions and a metric socket a 10 or 12 I forget which, to remove 2 bolts. I used a few short extensions that way it gives a little play on approach angle you need to go by and through the mounts. The second bolt I found it easier to remove the serpentine belt as the belt tensioner is in direct line of that bolt...after removing the belt let the tensioner down easy as it will go quite far and it will hit your cooling fan,,, and fingers in the way. I use a pipe over the ratchet for leverage and finger clearance... a lot eaisier if its a 1 person working on it, and no one to take off belt for you. Remove the second bolt and unplug the egr... it is electronic and about $150.00...an aluminum gasket is underneath.
Replaceing is reverse...but be carefull as the manifold you are bolting to is aluminum and the bolts iron, and you can easily strip the holes, I crossthreaded on myself, and it wouldn't tighten and seal the egr to manifold. I removed it and placed some washers to fill empty space and get the bolts tight. My friend has a blazer and same thing happened to him. After replacing this expensive, pain in the butt part my truck runs smooth , acceleration restored,and rough idle was cured, and no "service engine soon" light.Good luck!