I'm also glad you posted this. So often we tell people NOT to let dogs ride in the back of trucks...and everybody thinks nothing will ever happen to THEIR dog....Well...it DOES happen. And it's awful. Hopefully, someone will read this post in the future and make the decision to keep their dog safe.
I'm sure you feel lousy enough without any of us scolding you...and I'm VERY sorry Diesel was hurt.
Give the guy a scratch for me and keep us updated!