This wound up being a non-issue. There was nothing wrong with the transmission at all. I did something mildly stupid but at least had enough common sense to ask about it before doing something *really* stupid. Here's a free laugh at my sleep-deprived expense:
Broader did a fantastic job, but I mixed up linkages. The ISS were different between my old C6 and new C6, new one wouldn't accept an external NSS. So, I looked for alternatives and opted for a B&M shifter where the NSS and RLS are in the shifter and not on the transmission. I blame it on lack of sleep, but when I was setting the push/pull cable length, I said "Hey, I should make sure the shifter position matches the transmission position first before connecting anything and adjusting travel." Shifter was in neutral. Stupid me, I picked up the (new) kickdown arm and slapped it on to "make sure the transmission was in neutral." There was never an actual gear selector arm installed on the shaft, and I had a bag of about 10 different arms on the workbench...
Mr Broader sparked my memory when he asked which shaft was stuck. In a moment of clarity and embarrassment, I went back to the garage, pulled out the new gear selector arm, put it on, click click click just fine.
I removed the original post because I don't want anyone to think there was a problem with Broader Performance, they've been excellent.
