I have managed to do it a couple of times where you're able to get onto the back of the truck before Clementine leaves the yard, and I see it in runs people have posted, but is there a consistent setup to get it every time, or is it just luck?

Best way I've found is to cut the corner of the telephone pole at fence as much as you can. Closer you get to the pole the earlier you can hop on the truck. Also jumping towards the right when you hop down from the fence helps.