TXQH is right. The straight axle Dana's will have the number stamped in the webbing around the center-section. The D44 and D50 are twin i-beam or TTB, and only have one fill plug. Trust me on the TTB, it is much easier to get a cheap pump from the parts store, it comes in handy for draining the rear axle and t-case, too. If you have the Ford 8.8" rearend, you can take the cover plate off of the back, but then you need a new gasket.