Welcome to the Affliction.
Could be one of very many things including alignment: Tie-rod ends, draglink ends, kingpins, spring pin bushings, wheel bearings... I don't know what you have done so far, but I suggest you jack up the front end and put stands under the frame so you can inspect, shake, and wiggle everything to see where the loose-ness is.
Do you have a shop manual? If not, get one. Some are reprinted on the net.
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0301.htm Repair parts are available to completely rebuild your front axle and steering, some of them from NAPA and other local auto parts stores.
Good luck.
