Gunner (our younger boy) has Hip Dysplasia. So we too know about bad joints. We used to have our two pull sled, but once we noticed Gunner was walking weird, and had him x-rayed to be certain, that all came to an end.
But the Mal we know that is a retired weight puller, has no problems with his hips/joints. So I gotta wonder if over time while training the dog (if they don't already have joint problems), their joints get more condition to it. Kind of like how fighters condition their bones to be able to take/deliver more punishment without fracturing or breaking.
I don't know, I don't do it lol. But so many people weight pull their dogs I cant imagine every one of the dogs end up having issues. I love seeing working breeds working. I wish we could still pull sleds with ours. So Huskymom09, I find it awesome you do that with 2 of yours!