Why does the truck jerk while driving in four wheel drive?

Answer 1

Because as you turn corners the path and distance each wheel actually goes varies. In normal 2 wheel drive this difference in distance travelled by the rear drive wheels is accounted for by the action of the rear differential.

But some/many four wheel drive vehicles do not have an equivalent differential type action between front and rear wheels. (I drove Suzuki Samurais, I don't know about Chevy pickups specifically.)

The jerking and lurching can be intensified if there are "LOCKERS" installed...devices that work in the differential to limit wheel spin on one side when the other side still has traction.

When driving in mud, sand, snow, or other loose surfaces this is not usually a concern, as any slippage will be between the tires and the surface being driven on. But if driving on good solid traction surfaces, such as clear dry concrete or blacktop, the tires are less likely to skid on the surface, so things get bound up and jerking/bouncing can result.

This is why on older more primitive 4x4's, one should never engage the 4 wheel drive if not on loose surfaces. My brother once broke gear teeth in a 1969 Jeep by leaving it in 4 wheel drive on dry pavement. The tires had too much traction and the increased strain as a corner was turned was strong enough to break the gears in either the differential or transfer case (I forget which.).

Many modern 4 wheel drive vehicles do not truly lock the differentials or front to rear gearing, and are more tolerent of driving in 4 wheel drive on sound pavement.