A couple of guys already said it...... U joints!
My rear U joint started to “let go” with less than 50k on it on my old ‘06 RAM. Similar scenario, felt the vibration more when towing but it was still evident without pulling the trailer. Blocked the rear end up off the ground and crawled under it..... That’s when I discovered the bad U joint! I’m REALLY glad I caught it when I did as we wouldn’t have gotten too much further with that worn out joint! Doesn’t cost anything to look at your U joints, it’s a good place to start anyway.
Good luck!
Dan