When reselecting the LaserWriter 8.1.1 driver in the Chooser after using fax/modem software, an open Claris CAD document may shift up and left. It seems that Claris CAD is retaining the page size from the fax/modem driver and is not allowing the margins to be reset while the program is running.
The solution is to Quit from Claris CAD, reselect the LaserWriter driver and reopen Claris CAD.