I believe it's called a check spring.
There is a repair spring (Dorman HELP! 38435) that gets held on by one of those 1/2" head bolts.
The spring by itself is not available through Ford.
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.