the rear end is welded...not sure when maybe 3rd member?
![Huh??? ???](https://board.marlincrawler.com/Smileys/marlin/huh.gif)
If it's your driveline making the vibration take it off and check the u-joints or just have a driveline shop replace the u-joints.
I have never ran a welded rear end but maybe your tires are off balance or un equaly worn or you should check the tire pressure. Just some simple ideas.