Check the condition of the center link, tie rods, and other steering components for wear and looseness. Once you have determined that they are ok, check suspension parts, A arms etc. With a wheel off the ground and vehicle supported on stable stand or block (not cement) pull, push, wiggle top and bottom of the tire/wheel. Note if any excessive play. You will get a little in the bearings which is normal but more than that is a problem! While the wheels are off the ground check the steering by moving the wheel 1/2 turn either direction from straight ahead. Note any noise, resistance or play. If all that is ok then check the toe-in to make sure the front of the wheels are a bit closer than the back. Neutral or toe-out produce wander, hard steering and failure to center. Caster and camber also effect those properties. If you can't find the problem easily take it to an experienced front end man. Someone that remembers working on vehicles without power steering. They will all be over 50 years old.