Check if the bolts that hang the Steering Box from the frame are tight in place, just this weekend i repaired exactly this issue, ONE washer failed and all of them were becoming loose, glad i caught it on time.

 

on the pre 92 trucks there is a teflon bushing between the upper and lower steering shafts.
it is held in place by a small piece of spring steel. if the retainer falls out, the bushing falls out leaving you with about 1/4 turn play in the steering.
to test if the bushing fell out, have someone sit in the truck with the engine off but key turned to steering unlocked.
open the hood and grab hold of the lower steering shaft just above the rag joint. have the person inside wiggle the steering wheel side to side. look for play at the upper/lower shaft connection.
the bushing can not be bought separately from ford. if it is missing you will have to replace the steering column. it can be repaired with parts from a junk yard column, but it is a bitch to do. it takes me 2-3 hours to do it, and i know what i am doing as i have done it quite a few times.