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Beleive it or not, INNER spline engagement is only 0.4" on an early power-lock differential.

Off the shelf Yukon axles are actually 27 7/8", which is 1/8" shorter than they should be. If you remove the thrust button, you can run a 28 1/8" axle. Any thing longer will interfere with the internal cross shaft expansion in your sure-grip.

Keep in mind, the axles will measure 1/8" longer if you measure them diagonally.

BTW, I machine correct length axles for your application. E-mail me if you are interested.