You may wish to check out this post:
08-06-2013, 2109S Slide Motor Pulled Loose, posted by lswartz
Check out the photo in post #6
Before I installed a brace under the slide motor on our 21FBRS it would rock up and down about 2". With the slide pulled in the motor would rotate down onto the blue/cold water line, and probably gone to the floor if the line didn't block it. With the slide extended the motor rotated up about 2-3/4" above the floor.
That is the height of the brace I installed.
Thanks to lswartz and others for pointing out the problem/solution.