A few thoughts...make sure the shock isn't topping or bottoming out, and check clearances, the tire or wheel might be hitting it at some point in the suspension travel. Also, how stiff is the valving in the shocks you're using? Could be too much for the mounts.
I've had to put outer braces on those stud mounts before, just sticking out into the big wide world isn't the strongest mounting method. Even my IHC Eagle tandem rips 'em out!