From your signature, I'm guessing you can't use a crate (this is one area where I would recommend them).
My Moose-dog gets nervous in the vehicle. He doesn't like the "sleep guard" that makes noise when you pull over to the side of the road; he doesn't like lots of curves and such.
I try to get him to lay down as much as possible to minimize the amount of movement his body feels. Could you try using a seat belt harness with him to keep him in one spot?
I also try to comfort him when he needs it. In the clipper that's easy with just me as he can stand next to the driver seat while I pet him. For you that might mean having him situated as close to your spouse so they can pet him and talk to him while he stays where he is.
Since they used to be good riders, it may just be that they need time to remember the good of riding in cars. Take them for regular weekly trips to the park or the local pet store for a treat. After a few weeks, start taking them a bit further out.
And the main advice is to just slow down. Don't take those curves and bumps as fast as you do now - drop your speed about 10-15mph on them. See how that affects the dogs. Remember, you're sitting in a seat MEANT for your comfort. They are trying to fit on "furniture" meant for people not dogs. So taking a curve at a speed that is comfortable for you is likely too fast for them - their bodies are handling the movement differently on those seats.
