Thanks for all the support!
He actually did pretty well. Car rides for him are not fun, but he handled it in stride. He didn't even bark at either of the toll booth individuals!
He had a minor meltdown when the vet first came in (they had us in this big training area), but once we moved to a smaller room he just laid down and was so, so good. The vet said his reactivity is quite severe, which I already knew, but I was incredibly proud of his behavior.
His medications have been entirely changed. Previously he was on flouxetine and trazodone, now he's on Paxil and clonidine. No idea if it will be "better" but maybe it's a start. We're also going to be working on more obedience and hitting harder with some behavior mod.