Thanks Greg.

This particular Allison in my 2001 GMC used to have a more noticeable drain-back (slow engagement after sitting for a time), but that has almost all gone away with time and miles. Now, engagement appears normal after sitting for up to ~2 weeks or so.

Fluid level appears normal. Checked when hot yesterday. Was found to be at the marked hot level.

This truck is still running the original black NSBU. I haven't seen anything wonky with the shift display.

I'll be visiting a friend (transmission specialist) this morning. He has a scanner and wisdom. I'll report back as to what codes were present along with anything pertinent.

Gotta say though that my heart sank when the SES lamp popped on, given the head gasket replacement project was completed just a week ago. I was a little relieved that it was a P0700...