I've got a '93 Pickup, 22-RE, 2WD, that has either an erratic (constantly up and down) or inconsistent idle speed after the engine warms up. Under the hood, all the hoses look ok, so I don't "think" it's a vacuum leak. Acceleration is just fine; no backfires or sputtering. I am wondering if a bad O2 sensor would cause this (???). I tested the resistance on the sensor terminal using my digital multimeter and it read between 6.4 and 6.7; the FSM says it should be between 5.1 and 6.3. Do y'all think the O2 sensor could be the culprit to my idle speed woes?

I would first check the Throttle Position Sensor:
http://www.4crawler.com/4x4/CheapTricks/TPS/index.shtml#Troubleshooting

Commom symptom of a mis-adjusted TPS IDLe setting is:
* Varying idle speed

As 4Crawler mentioned.

TPS

Idle speed too high,

or, believe it or not....

Coolant needs topped off.

One or more of the above will fix you up.

PS, this happens while stopped, like at a stop light or something huh? With the brake applied.

I am going to say coolant, unless this is happening when your hitting the brakes. then high idle

Wow, I'm shocked...my coolant was close to a quart low. I filled up the radiator to the cap; no more idle issues. I'm just stunned - I never would have thought that low coolant level would affect engine idle. Thanks a million guys!

Cheap fix huh? Some of your sensors weren't getting a good read on temperature of the engine. Not enough coolant to pass thru or over those sensors I guess.

Now, where did the coolant go?
Keep an eye on the coolant level, and let us know if you need to keep topping off.