This is not an issue.

The transmission like any other has some internal friction loss, or churns some oil, either or both can result in the output shaft gently turning in neutral with no load on it.

Install the driveshaft and continue on.


•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-