When I had my 3 piece driveshaft rebuilt I recall there was a forward mark on it. For those following along at home only the 1991 350SD and 350SDL have a 3 piece driveshaft. 1990s do not.

Perhaps this video will be helpful? https://youtu.be/7xqeine6rs8?t=118

The 2 centering bushings are different lengths. The shorter one is the front / transmission side.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.