Unusually high idle speed

My wife has a 1999 4wd Blazer. Over the past month or so, I've noticed that at times it has an unusually high idle speed. After it warms up a bit, this stops. When it's in park, the RPM's sit about 500-800ish, which is about par with what my 2000 GMC Jimmy would do. In drive, I work at keeping my RPM's below 2K, which is not difficult. I got it out onto a decent straight road and from a dead stop let the idle speed take the truck. It topped out between 35-40mph. It does this when the temp outside in the morning was in the 60 degree range, and in the afternoons when the temp is in the 80 degree range. After I drive around for 15ish minutes, it stops doing it.

Truck has 97K miles. Transmission was rebuilt approx. 20K miles ago, and have replaced multiple minor items(alternator, power steering pump, battery).

Any ideas would be greatly appreciated. Thanks in advance!

(small side note/question-- There is an odd "clunk" when i'm driving and turn either left or right. Any ideas on that either?)

I would get it on a good engine capable scanner and check the actual and desired rpm readings.
And go threw the iac motor test in the scanner.
Also read out any codes and post back code no.
4W or 2W?
On the front end look it over for anything worn/lose.
Check for good lube of all stuff that you can lube and check and lube rubber bushing and sway bar links and all bushings.