Jeep began offering Eaton mechanical cruise control around 1976. Going that route requires finding an intact system that can be reconditioned and put back into service since nearly all the parts are no longer made.
It's not very hard to install. You just need all the parts and you need to have enough mechanical aptitude to open up the governor, understand how it works, and fix it if needed, because the valve body seal inside it is probably toast.
'72 Jeep Wagoneer Custom, 360 V8
I love how arguements end as soon as Ristow comments. Ristow is right...again.