O2 Sensor codes-88 Blazer

My hubby has an 88, 4.3L. We just got back from a repair shop about an hour ago after having the fuel filter replaced, and it did no good. So we followed an AF suggestion about checking the codes on the ALDL, and it threw up codes 13 and 44, both dealing with the oxygen sensor. I saw several good suggestions on here about checking the manifold intake wire (we could take the throttle body off, but how easy is the wire to find?) and spraying it with carb cleaner, as well as checking for clogged fuel injectors, to replacing the cat converter.
What would be the best suggestion to see why it's running so rough without replacing a dozen things on the engine? What other information would be needed to make that determination? I just don't want to run it up to the mechanic's every day and spend hundreds of dollars only to find out it might have been a $10-10 minute fix. Thanks!

Clear codes and unplug 02 sensor for a test and see what it does.
Also check egr valve for being stuck open.
Your code 44 indicates system is running lean.
Make sure you have good fuel pressure.