Fixtures, maintenance assemblies, and methods for maintaining a wind turbine gearbox are disclosed. The gearbox includes a sun pinion and a bull gear surrounding the sun pinion. The sun pinion and the bull gear each define a central aperture therethrough and have a central axis. The gearbox further includes a casing, at least a portion of the casing coupled to the bull gear. The fixture includes a frame member and a shaft extending from the frame member. The shaft is insertable through the central aperture of the sun pinion. The fixture further includes an attachment mechanism removably attachable to the sun pinion. Upon attachment of the attachment mechanism to the sun pinion, the fixture is configured for moving the sun pinion along the central axis of the sun pinion.