Involvement with the Eagles
Best known for his hits "Running on Empty" and "Somebody's Baby," Jackson Browne made friends with Glenn Frey back in the Longbranch Pennywhistle days. At one point, he lived in a small duplex underneath Glenn and his then-roomate, J.D. Souther. Glenn heard him composing Take It Easy on the piano as he laid in bed upstairs. Browne was frustrated with the song and was going to give up on it, but Glenn thought it deserved another chance. He filled in Browne's lyrical gap with the line "It's a girl, my Lord, in a flatbed Ford, slowin' down to take a look at me" and recorded it with the Eagles - and the rest is history.
Doolin-Dalton was inspired by the real-life gang. Glenn had received a book on the gang for his birthday before he hit it big. He and his friends, including Browne and Souther, were fascinated by it and its graphic photos of dead outlaws, which would inspire the Desperado album's back cover. A couple of years later, while he was hanging out with them along with Don Henley, they started taking about the gang again and the idea of the anti-hero. This also inspired James Dean, which they worked on at that same time.
Browne appeared as a special guest on the Eagles' 1974 Don Kirshner Rock Concert TV special. He shared leads on Take It Easy and the Eagles backed him for an early version of Your Bright Baby Blues.
You can read an article where Jackson Browne discusses the Eagles a bit here.