I say scope or mount. Most likely scope.

Just cuz it worked on another rifle, does not mean that the scope didn't get damaged during the process of switching it. Or it was close to damaged already and the recoil knocked the crosshairs off their mounts.

Put the scope back on another rifle and see if you can get it to hold zero. If it does, then your mounting system is F'd. Rings don't tend to move once mounted. You usually can see any damage that would cause that kind of movement. The mount could easily seem tight, but with the recoil of the shotgun, move just enough. Don't take much movement in a mount to throw rounds that far off.

Try the scope first though. Let us know, I'm interested to see how this comes out. Good news is that it's a Leupold. Lifetime warrenty baby! Those guys are legendary for their excellent customer service.