More than likely its the steering box itself, as they age (which yours definatley is at 263k) it simply gets play in it from wear. You'd have to get it rebuilt most likely to get rid of the play. It may be possible to adjust some of it out with the adjustment screw, but its unlikely at that mileage that it'd make much difference.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)