In principal wheels can be rigid bodies without soft physics, but to make them stable and avoid jittering you need to process collision in a custom way. I’ve seen tank implementation where collision of drive wheels with the ground was handled as a spring contact not as regular rigid impact. Spring force itself is later used in friction calculation. The amount of spring force is proportional to penetration depth during collision. Pushing wheels above the surface is handled not using impulses but the spring force itself.
I have zero idea how you could implement this with Physix in UE4 as collision processing is handled on a low level and in UE4 you get only collision response, but what you need is to override the collision calculation itself. In demo with tank which I’ve seen all physics where custom made.