I had a '74 F-150 4x2 that did the same thing. What the dealer did was install a steering dampener on the linkage, a thingy that looks like a shock laid sideways. You don't say if your is 4x2 or 4x4. If it is a 4x4 it probably already has one that has worn out.
BTW, you'd get better response to this in the '94 - '97 forum.