On a brand new truck there would be some movement you could probably see, but our series of trucks are now feeling their age. With all the moving gear train parts, some wear on each can add up . As Mr schurkey stated a lot of wear in the connected axle parts can cause excessive amount of ring and pinion movement, ( drive shaft movement) as it shifts in to a drive direction. At idle the engine in drive wants to move the truck. So wheels with brake on then shift to drive the power takes up all the drivetrain slack.