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Bump steer is an issue all automotive engineers have to deal with. Even the mighty Dodge Viper suffers from bumb steer. I nthat case its an issue with the suspension geometry not uncommon to that class of car.
On my Race Tin Trials Truck bumpsteer is worst when one wheel hits a bump. The return spring is not strong enough to prevent the wheel from being deflected by the bump force that is pushing it both back and up at the same time.
The play in the steering system exasperates the problem. Even though you have a servo your steering linkages likely have some play in them which is required to prevent binding.
As a result the wheels turn. On this car they are moving straight up and down on a vertical shaft so the suspension does not travel in an arc.
If your truck uses a rocking solid axle your wheels will move in an arc but the steering arms move in the same arc so I don't think that is the cause in your case it is more likely the bump force moving backwards against the direction of travel that is the main issue.
You can check this by seeing if the bump steer is worse when the wheel goes over a square edge bump compared with a shallow angle bump.
On the RTs and some of the other 1/28 and 1/64 mini rc cars some people use heavy grease, oil soaked tissue or wax to take up some of the play or slop in the steering system. Some people also wrap the shafts in the system in tape to take up slop.
It will take some trial and error to determine the amount of slop you can take out without causing binding in the system.
If you can post pictures or even a video clip of the problem the members can probably help more.
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