The rear mount consists of the upper bracket that bolts to the engine timing cover, and the lower bracket and backup plate that sandwich the sheet metal of the van at the rear behind the bumper. Bumper removal is required for install.

The original cantilever mount setup is fine without the rear mount *if* the bellhousing is free from corrosion and in good shape. If the bellhousing condition is questionable, or heavy duty use (hard acceleration, off-road etc) is indicated then the rear mount is a good idea.  The rear mount also allows "tuning" of the transmission and engine mounts in terms of loading for complete control. Lowered NVH (noise vibration and harshness) can be a benefit by playing with the multiple adjustments possible.

Read the install doc here