For emissions reasons the RPMs should hold around 1,000rpms until speedometer is below about 5MPH.
On manual transmissions RPMs should hold about 5 seconds or so between shifts, this is an MPG and emissions thing
And yes, this would only happen below 3,000rpm because IAC Valve can only hold RPMs as high as 2,000 or so
After engine is full warmed up shut off the engine and check throttle return manually.
Open throttle and let it close, making sure spring return is OK, no stickiness and it closes all the way each time.
Then start engine and repeat same test, spring needs to be STRONG to close throttle plate against air flow, there is 20" of vacuum inside the intake trying to suck throttle plate open, and any extra air = increased RPM
Air is RPM, not fuel, add extra fuel to an engine and you get a Flooded engine, add extra air and you get higher RPMs.
There is no "gas pedal" its always been an "air pedal" on gasoline engines
Vacuum leak test
Engine warmed up and idling, unplug the IAC Valves 2 wires.
IAC Valve will close all the way and RPMs should drop to about 500, or engine may stumble and stall, either is GOOD, it means no vacuum leaks.
If idle stays the same then there is extra air leaking in.
4.0l SOHC had a common issue at PCV hose elbow, it would crack, on the under side so you don't see it, so smaller air leak that will get bigger.
Since you did some engine work, and noticed the problem afterwords, an older brittle vacuum hose could have cracked when moving it around
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