Can't remember my dog stuff like I used to but, I think a trotting dog doesn't have all 4 legs touching the ground at once. That may be why he trots better than walking.
The trot is a dog's most efficient gait; when it has a lot of ground to cover, a wild canid usually defaults to the trot. Basically, the animal tenses the back muscles, and bounces from one diagonal pair of legs to the other. The process of "unloading" one pair of legs transfers part of the energy to the muscles supporting the other pair, kind of like sets of springs, so it doesn't take a lot of energy to maintain the gait.
It might seem that the bouncing of a trot might be jarring enough to be even more painful for Ben, but it's possible that, because the muscles along the spine are tensed up, the damaged joint is supported better at a trot. Walking, most muscles would be relaxed much of the time, which would allow more sagging and flexing in the painful area.
That's my theory, anyway.
DL, I'm sorry to hear about Ben.

Knowing that a beloved animal is getting older, slowing down, not as game as it used to be - these things we can accept, but seeing one in pain just tears at the heart.