This suspension replaces the solid axle in the original package. The change allows a greater weight to be carried than was possible with the lighter springs used on the solid axle design.
It requires 3 (or 5, depending on weight) x 85mm longit. springs/shock absorbers (see photo) per end. They are around $4-5 each from hobby stores.
The .3dm available for download with the vehicle now includes this suspension and the original solid axle design.
You will require the springs (these are easily purchases for RC model shops) and 3mm and 5mm bolts and washers.