Driveshaft Engagement
Dr Diff advised me to bottom it in the transmission and then slide it back out 1 to 1 1/4 inch. Make sure the seal isn't holding it back from bottoming fully.
On a side note, verify your engine/transmission center-line angle is in proper orientation to the pinion shaft center-line. They should be parallel to each other after the nose of the rear-end lifts under power. (pinion should be a degree or two nose down when there is no load is on it).
https://www.hotrodhotline.com/md/html/drive_shaft_harmonics.php