The preferred disconnect is at the speedo housing. Someone in the past has tightened the fitting with a pair of pliers. You will have to use the same to get it loose. Once you get that first 1/2 turn the rest is finger tight. Do the job right while you are under there. Pull the inner cable out of the housing. Take some PB Blaster or similar and using the tube nozzle spray a good amount through the empty case. Then let it dry or blow it out with air. Go by your local FLAPS and get a tube of speedo lube. Lube the cable and slide it back in the housing then reconnect back to the speedo. One hour easy job and the problem is fixed for as long as you will own the truck. Good luck.
