1970 C10 Speedo Cable Replacement

[rmawyer]

Speedometer needle skipping. Truck has a 68 inch cable that goes from speedometer to a coupler (maybe for cruise control?) in engine bay. There is a second lower cable from same coupler to transmission that is about 55 inches long. Home mechanic friend used drill to show upper cable working; subsequently determined lower cable is broken, needs replacement. Not having any luck on tracking down a replacement cable. I may not be using the correct cable name in my online searches. Any ideas on where to look? Also, is the cable unique because of the cruise control? As always, thanks for any insight shared.