1994 S10 Blazer Not running smoothly

My problem with my 1994 Blazer S10 4x4 is when you are driving it at 40mph and above it runs roughly. It seems like it's hesitating, missing, or not getting gas. It also runs rough while it's idiling. The engine is a 4.3 vortec engine. We have replaced the spark plugs. Also the anti lock light stays on all of the time. Any suggestions.
Debbie

Was it doing it before new plugs?
First thing go around and check for a missing cylinder and make sure it has good hot blue spark to all plugs.
Could be bad plug wire or you cracked or mashed a plug to gether.
If you have a miss plull your plug and recheck it.
What kind of plugs did you feed it.
Check all vacume lines and hoses may be one got knocked off.
Any check engine lights? How is the fuel filter?
Also a sticking egr valve will make it run rough to.
Let us know what you find on the engine miss.
MT

Yes, it was running like that before we put in new plugs. Could you tell me where the EGR Valve is located, so I can check that. And how do you take it apart. It's a 4.3 Vortec engine.
Thanks,
Debbie

What kind of plugs did you feed it? Any check engine lights?
Any cylinder missing? Good hot fire to all plugs?
All things you need to check.
The Egr valve is on front of intake. Looks like a pint can with wires running to the end of it. If it is acting up or stuck you should be getting a code.
You can check for carbon stuck in the plunger by removing it and looking a the end of it. But be carefull with the 2 bolts that hold it on they may be frozen up and or strip out or twist off.
If you remove it and check/clean it get one of the gaskets like tomco or AZ that has a screen in it.
The carbon ball sticking under the egr valve plunger is a sore spot but the gasket with screen will help. Also when the valve is off start it up for a second and blow carbon out of intake.
MT

The Plugs are bought from walmart, they start with an A. And no there isn't any check engine lights on. The only thing that's lightning up on the dash is the anti lock brake light. There doesn't seem to be any cylinders missing and there seems to be good spark to all of the plugs. In fact the plugs weren't bad when we changed them. You said that if the EGR vavle is acting up then I should be getting a code. Where does that show up and what is the code? I also have another problem, it's with the four wheel drive. It has a switch on the dash. You can put it in four high and low both worked, but now it will go into four low but not into four high. On top of that if you drive it in two wheel at 50 or higher it seems to kick into four wheel drive, but then you slow down to 40 or lower it seems to kick back out. Any suggestions on what could be wrong would be appreciated.
Thanks.
Debbie

The egr valve sets a check engine light aand you can pull codes and get the code.
I asume you put auto light plugs in it.
That is a no no. Throw them in the trash can and get a set of ac delco sparks that the book calls for on your engine.
And go from there.
Check for good hot blue spark to all plugs .
Check that fuel filter and fuel pressure to.