1. Did you completely GREASE the Spline-ed Shaft central portion of the D.S.
It should be able to compress enough to allow you installation.

2. You are aware of the "Centering Bushings" at either end of the D.S. ?
They fit into the Yoke @ the Tranny end and whatever is @ the rear
that the Flex Disc attaches to (Another Yoke) They must be greased also.

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'84 300SD sold
124.128