I have no advice, as hubby isn't allowed (nor would he anyway) put the dogs in the bed, so they have to sit behind his seats in the space back there..which well now that they're bigger of course will fit only one lol (small truck). BUT, I would think they would get beat around there a lot turning and what not, falling down and maybe getting hurt? I know you said a short lead but when I was a kid we had 2 Goldens and were moving and my Dad took our dogs to the new house in the back of his truck.. I forget where he tied their leashes but when he got to the house one of my dogs- he was very large..decided he wanted out and over the side he went only to get hung up on his leash and was choking..thank god we were standing right there and my mom started screaming for my dad to come help free him. It was awful. We thought for sure he was going to snap his neck on the way down, but thankfully it ended up okay.
I'd be entirely too paranoid to ever have them back there. What I never understand is people that just stick them in the back and expect them to stay there. I'd never trust a dog to do that. The crate idea sounds much better!