I noticed on the last few drives with my 03 disco II that there is a noticeable bit of play like one of the steering knuckles/ball joints is showing wear. ie: when attempting to keep the truck
centered in a lane I'm having to move the wheel back/forth a bit to keep it on a straight line. What would be the most likely culprit to check? Mileage is 71K. New tires just installed.

 

The steering box play is adjustable. Procedure is outlined in the RAVE (p.57-17).

 

I did this on my 87 ford f150. Just not sure that is where the play is. I can definitely feel something catching like a joint or knuckle that is rotating as you go from
one direction to another.

 

I used to have a massive play (1/8 turn easily) and the steering wheel would turn just from driving over a pothole. So this was my procedure: replace all ball joints: the ones on the panhard bar were really worn, and drag the link a little less - don't bother with the replacement parts, just get the bars, virtually the same price. Get the wheels aligned. Dot the box adjustment per mollusc comment (if you look up youtube videos, search for four-bolt Defender box adjustment). If you don't have rave all you really need to do is loosen the locknut on top of the steering box, and tighten the stud little by little, until the play is minimal.