5.0SC V8, and 6 speed ZF. 60k miles. I noticed on (very) cold starts, when I shift from D to R, or R to D, the engine RPMs drop quite a bit for a split second and I have once or twice gotten a stall. I have to do quite a bit of this D-R maneuvering getting out of my driveway which is slanted and has too many cars. Everything is perfectly fine once warmed up.

My car got a dealer transmission service 5,000 miles ago, prior to my ownership. Should I service it again to be sure it was done right?

silvertones, I have the issue with my car. It seemed to get worse right after a fluid change (installed the ZF Life XXXXX 6 fluid, whatever it is called). I am not sure if it is simply the torque converter not releasing as much as it should or what it is. Something that I have found that I can do is shift into neutral as I get down to a few mph to then roll to a stop and then be gently on the brakes as I shift it back into gear to allow the car to start rolling as it drops into gear. If you find an answer to this, please let me know. My car is not throwing any codes, just has that "I want to keep rolling" attitude on a cold morning like you are experiencing.

Interesting, I haven't had it happen again yet but I have noticed that when I shift from R to D, I can tell it goes to 2nd gear before dropping to 1st. On a cold start, and on my inclined driveway, that shift to 2 really drops the RPMs low. Perhaps it is a programming issue

Not 100% sure if it is the case, but the cracking/leaking mechatronics sleeve(s) have known to cause issues on ZF6 transmission. If they leak, there are not enough oilpressure to operate/release clutches etc.
Check this: