Attach a vacuum hose to the front axle shift motor (I don't have a diagram handy so not sure which port, but I think it's the back port, furthest from the linkage) and roll the car slightly till you hear the axle click engaged (or turn the front axle yoke by hand with the front driveshaft off till you hear/feel the click as it engages or at least stops being movable by hand if you don't hear it)
Leave the vacuum attached and drive onto the ramps ready to lock up.