You've already rebuilt the trans and now it won't spin? Your clutch packs are not lined up correctly. 90% of the time that's the issue. Something is not lined up properly.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k