RE: Replacing front axle shaft on 2002 F250-
But as stated, if its overdue or close for a sceduled service interval, than its a judgment call...
If you raise the truck on a level lift, or jack up/tilt just' the side that the axle will be pulled from, then very little gear lube will leak down the axle tube.. Also, make sure that the axle tube is relatively clear/cleaned out on the inner bore if the outter dust seal has blown (usually are) from dirt/sand ect remaining prior to assembly.. They can collect a decent amount if the seal has been gone for some time.. Otherwise the axle splined end can move the debris into the inner axle seal 'pinch' sealing surface and leak, and possibly into the rear diff. as the replacement axle is pushed into place. Check the ball joints for loose while the axle is out as well, if you haven't already.. Top off as needed, if not replaced..