Now that I think about it, vibration only during deceleration should be in the drive-line. You say you have had the rear apart (checked pinion and side bearings). This is a 56 integral rear, correct? It hasn't been upgraded?
If tire balance/out-of-round/ply separation) it should do it at most all times...
I added your 2nd photo (much better appearance
) in the original post.