Brian, your '93 isn't the 5.4, it's the 5.8.
(Same engine my '95 has.)
The IAC is the "Idle Air Circuit" valve.
It's accessed on the front of the motor (under the hood) --- once you remove the air-cleaner assembly it becomes more visible. (the IAC is bolted to the side of the throttle body, I believe.)
I replaced mine to help smooth out an increasingly erratic idle as well. (It did not fix it.) What finally cured my bad idle was replacing the EGR valve...and most importantly the EGR sensor
. Keep those two items in mind! And good luck.
Let me know if you need any more help with your engine, I've spent a heap of time (and cuss words) getting mine to finally run right. If you're going to get in there and replace the IAC, you should also replace the TPS (Throttle Position Sensor) as well.