Basic driveshaft elements include, from left, a slip-yoke, a universal joint, and a driveshaft. The slip-yoke rides smoothly in and out of the transmission as the rear axle articulates with road conditions. Universal joints allow articulation and power transfer. As a rule, they're sized the same fore and aft in stock applications. Slip and pinion yokes vary according to application. There were a lot of running changes during the first nine years of Mustang production. Expect to see variations throughout a given model year.