Is your Chilton's manual Procedure similar to this?;

5.8L V8: 1988–89 Truck and Van
To adjust idle speed:

1. With the engine off, place a 0.030-inch feeler gauge between the throttle stop screw and
the throttle lever.

2. Disconnect the SPOUT connector at the ignition module and verify that base timing is
within ±2° of specifications. Refer to the VECI decal. Leave the SPOUT connector
disconnected until adjustment is finished.

3. Disconnect the ISC air bypass solenoid.

4. Place the transmission in neutral or park. Start the engine and let it idle for 2 minutes.

5. With the transmission in park or neutral, turn the throttle stop screw, to set the idle speed to specifications:
– 780 ±50 rpm for 1988 models with AT
– 730 ±50 rpm for 1988 and later models with MT
– 780 ±50 rpm for 1989 and later models with C6 transmission
– 730 ±50 rpm for 1989 and later models with E4OD transmission

NOTE: If the screw must be turned inward, stop the engine. Make an estimated adjustment, start the
engine, and repeat steps 4 and 5 before continuing.

6. Stop the engine and disconnect the battery for at least 5 minutes.

7. Remove the feeler gauge from the throttle stop screw and reconnect the ISC air bypass
solenoid. Reconnect the ignition SPOUT connector. Verify that the throttle is not stuck in
the bore and that the linkage does not keep the throttle from closing.

8. Run the engine and let the idle stabilize. Snap accelerate and let engine return to idle.
Lightly depress and release the accelerator and let the engine idle. See the VECI decal for
specifications.

NOTE: If idle speed oscillates or is erratic, the throttle plates may be open far enough to allow canister
purge flow. To verify the condition, disconnect the canister purge hose and plug it. Idle should
stabilize. If purge is present at idle, the throttle plates must be closed until the rough idle stops.

9. With automatic overdrive (AOD) transmission, check throttle valve (TV) cable adjustment.

 

It's similar let me find it

 

 

Try replacing the idle air bypass valve on the side of the throttle body also you can get a EGR delete plug that plugs into the wire harness and the will get rid of any EGR related problems. If the EGR position sensor is throwing a code it can screw up the idle. Also when you do anything with replacing or changing sensors unplug the battery and pull the headlight switch for 2 minutes to reset the computer.

 

this was a year ago, but I'd think the EGR still not fixed , like asked to do in 2nd post by 89F2urd