Everything was fine cruising down the highway until we came to some traffic nearing Old Lyme. The trans downshifted really harshly and I definitely took notice. Then when traffic moved again, it seemed like it was stuck in 1st gear. I kept winding it up to like 4,000 and letting off trying to coax it into shifting. Eventually I got it to shift, seemingly near 5,000. Yikes. And what it actually did was a very harsh 1-2 and 2-3 right next to each other. Which feels terrible. Slowed down in traffic and it did it again. Great, now the trans is failing, I figure. 91k on it...
Anyway, I drive the rest of the way like this. I figured out at one point that selecting 2nd actually starts it in 2nd (forcing an upshift) which gave me a manual 1-2 shift. So that's something.
Some online researched revealed a few common A4LD issues. I zeroed in on one of them. Apparently this transmission, and the C3 it's based on, uses a vacuum modulator valve (in addition to the TV cable) to somehow control shift points and line pressure. I also learned it has a computer controlled 3-4 shift. Incidentally, the 3-4 shift seemed fairly normal and it would lock up fine. Trans fluid inspection revealed proper level and good condition fluid.
I put the truck up on his lift (handy) and found this:
So....I fixed it. With some 1/4 inch vacuum (or fuel) line.