A header drivetrain coupling system including a harvester driveline mounted to a harvesting vehicle and a header driveshaft mounted to a detachable header. The harvester driveline is supported for rotation at an inboard end on a feederhouse and at an outboard end on a header tilt frame even when the header is detached therefrom. The harvester driveline comprises a telescopic portion which permits a coupling member on the outboard end to be moved into and out of engagement with a coupling member on the header driveshaft. A sliding support assembly is provided to support a bearing housing which holds the outboard end of the harvester driveline on the tilt frame. An actuator is configured to move the bearing housing with respect to the tilt frame to selectively engage and disengage the coupling members.