Try the easy part first. Disconnect the cable from the spedo,
remove the inner cable and apply some white grease or a suitable light lubricant. It could very well be sticking in the housing causing the problem.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine