Possible bump steer?

I would get the swivel pre-load checked. If it's never been done I'd stake my mortgage on it being ready for reshimming. Not horribly expensive and will transform the on-road manners.

At the same time, for the small cost, I'd fit a decent new steering damper. A ten minute job, just make sure you prime the damper before you fit it (instructions will be included, but you basically compress and extend it three times or so).

Almost certainly the swivel preload, exacerbated by the oversize tyres, although these are not large enough to pose a real problem. This symptom is typical of inadequate preload.

But I would also check the tie rod and drag link ball joints and the panhard rod bushes, and the wheel bearings.

Also, what's the offset on the wheels? If they're significantly different from standard that could be causing this too.

Before you start pulling things apart have a look for something really simple - mud stuck to the insides of the wheels can put them out of balance and cause similar problems as you describe.

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