I saw a forum post somewhere a few weeks ago that said it's common for these transmissions to shake like that and can be fixed by overfilling the fluid by about 300ml (1/3 qt). I didn't pay too much attention since my transmission started doing it right after I did a long road trip with a trailer, and the fluid level hadn't changed in a while.

I decided to put a little more fluid in my transmission this morning and drove around for about an hour trying to get the shuddering to come back, but it didn't. Too early to say it's fixed, IMO, but I'm going to keep an eye on it for a few more days. I'll let you know if it comes back.