The thread youre probably looking for refers to the electronic speedos in the 92+ trucks ("whats a psom and how do i 'p' it", in the tech write-ups section).
Since yours is pre-92 it should still be using the mechanical speedometer, in which case you need to change the gear on the end of the speedometer cable, where it fits into the tailshaft of the tcase. I dont think there is a write-up because it is so easy to do that you will spend more time gathering the 2-3 tools required than you will actually changing the gear.
Basically, follow cable to where it enters tcase. Remove small bolt and pull bracket away. Cable will now pull out of tailshaft. then pry off the the little metal clip on the end of the plastic gear, and slide the gear off the end of the cable. All youll need is i think an 8mm wrench, and a small flathead.
In your case changing either tire size or axle ratio will affect the speedo. You can get new gears with a different # of teeth online but its easier to just get them from your local ford dealer. Difference in cost is minimal.
Read
this thread by Ryan in the Tech Write-Ups section, and it will tell you how to figure out which gear you need.
