Based on all of your #6 post, the trans output flange has worn out splines.


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Engine on, shift lever back in park: clicking noise

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The clicking noise is from the parking pawl trying to engage the parking gear. ( think driving down the road then shifting into park. . . . )

Pull the drive shaft , output flange nut and remove the flange. It would be a good idea to pull the til housing and inspect the ball bearing / clean up metal shavings. This type of repair has been a few times on the Tech , Diesel and SL list so a quick search would net some info.