Hi Dave,
The swivel preload is the amount of pressure the swivel pins are pressing on the swivel ball. There are shims which are added or removed, beneath the swivel pins. The process requires disconnecting ball joints from the steering arms at each swivel ball, and with a spring scale measure the amount of tension required to move the ball. If it moves too freely, then the top pin is removed and a single shim is removed, replace the pin and remeasure. If more shim removal is required, it may be necessary to remove one or more from the bottom pin also, so that you balance the amount removed between top and bottom pins, until the ball resists movement.
The swivel pins fit into tapered sockets recessed into the swivel ball, and if the tension is insufficient, than the pin becomes loose, in the tapered socket, allowing sideways motion (and vibration) It is necessary that you have a workshop manual in order to perform this procedure, as it gives tension amounts and location to attach the spring scale.
It's a time consuming procedure, about an hour or more per side, but fortunately, not one which is done often (60,000 miles)
Before you begin to make your measurements, the pins should be inspected for uneven wear, as both they and their sockets can wear out if run too loose, too long.
Hope this helps.