You put in a new DS, so that answers the bad U-joint possibility. Did you do any work on the rear suspension, leaf springs or shock area?

I did all that last year and now have a new vibe that is prominent at 50-60 but is not present when above 60 or below 50. Just in that one speed range.

My best guess on my problem is that the new rear end parts have caused a previous problem in the tranny/engine area to become more pronounced now.