I've had similar problems with my Max. And we couldn't just pull over and go back home. We had a 10 hr drive each way and the entire way there, he panted, licked, whined, was hyper, everything but puking (thank goodness!)...when we'd gone before, he was FINE...a little angel; he slept most of the way and back and we had no problem (he was pretty young then though). So we kept thinking maybe he had to go to the bathroom and kept pulling off at rest stops to let him out, but he wouldnt' go. Then we thought he was hungry...nope, he wouldn't eat. It was just awful! He was FINE once we got out of the car. Once we got to our destination, I went to the health food store after getting online here to see what other people had done for their dogs, and got some Rescue Remedy. Well, I decided to test it out on him a few days before we got into the car to come home, just to make sure it was going to work. He started hallucinating

. Luckily, my dh was OUT at the time or he would have been FRANTIC. I figured it would wear off, so I took Max out for a LONG walk and sure enough after about an hour, it did. I went to Petco after that, and they suggested a pill called "Content-UM"....I gave him half a pill (they're hard to break in half too) in a treat, but I had to give it to him about every two hours. It seemed to help some, but once it got dark, he was all nervous again, so maybe it was just a coincidence.
Regardless, if you decide to go with the Rescue Remedy (it DOES work for a lot of other people's dogs), check on the ingredients; one of the preparations contains a lot of alcohol and the other one doesn't. You want the one that does NOT contain it. I'm going to go get some of the "Quiet Time" and see how he does with that...thanks to whoever suggested it!