I have a 1990 f-150 4x4 with the inline 6. The speedometer quit working about 2 weeks after i got it and im trying to figure out whats wrong with it.

 

Speed sensor is located on rear differential. There's a chance that it gone bad.

 

Being a 1990 you have the speedometer cable along with the abs sensor in the rear. It is common for either the cable to break or the gears in the transfer case to strip, as they are plastic. To check the cable have someone watch the speedometer and spin the end of the cable that goes into the transfer spin the cable both directions, if the speedometer move then the gears are stripped. If the speedometer does not move than more than likely the cable is broken. I don't know if 1990 F150 used the add on speed sensor for cruise control or not so you might have two cable to contend with.