Try taking a wheelbarrow, or a hand-truck, or a cart of some kind, and fill it with stage weights and other debris.  Then find an old area rug from your prop room and loosely roll it into your "body".   Then have a couple guys push the heavy dolly over the carpet, on cue.   Should make a nasty thud!  

 

Or use two rolls of carpet to create a nice double thump, like wheels of a car.   Any other junk you can throw in the cart would be good for some extra clank and squeek along with your thumps.  (And I'm sure there is a ham on the crew who can make a pathetic "yelp" and "fading whimper" as the dog is hit).

 

As with any backstage SF/X, it either needs to be done just out of sight in the wings in order to be heard, or else you will need to cue a backstage mic to pick it up better.   If your stage floor is too solid to resonate much sound, then try doing the same routine on top of a wooden platform.

 

Good luck with whatever you come up with.  -Dana