very common bias plate or 1 of 2 plastic rings break off on gearbox end of the gear stick you can replace the whole of the gubbings for about £12 and a hours work dead easy done loads and will make you feel like you have a new box
99% you will find its the preload on swivel pins as said above panhard bushes will make it wander but shakes after hitting a bump or hole will be the swivel pins