Welcome aboard, Idaho!
A few possibilities, in order of commonality:
1 - Throttle cable linkage binding. Easy to test, pop the hood, run engine (by hand) at 3-4k RPM for a few, then drop the linkage. See if it remains at high idle, and if so, try to manually decelerate it. If that's the fix, lube the cable and throttle linkage.
2 - AFM contact points can get funky and sticky. You may have to do a search on the process for cleaning them, it's been over a decade since I messed with one.
3 - On deceleration in the L28, manifold pressure surges and causes the RPM's to stay high. The factory installed a "Boost Controlled Deceleration Device" (BCDD). Allegedly, they have been known to fail. I have heard that some Z owners remove this piece completely with good results.
4 - Vacuum leak (although this should maintain consistent high idle, not just after stopping).
Keep us posted!