When you add extended travel shocks to the rear of a 4Runner/GX460 (2010- Current with KDSS), your droop travel is limited by the trac bar contacting part of the sway bar links. These drops spacers change the rotational axis points to allow full clearance of said contact.
If you are not 100% familiar with your setup, follow these steps to determine if you need KDSS rear drop spacers. Get some wheel chokes under the front tires. Jack up the rear of the vehicle off the tow hitch so that the rear suspension can droop down. Once the tires are off the ground, crawl under the vehicle and get really close to the trac bar and sway bars. If you see any contact, then that means the droop is being limited by the two components crashing into each other (see pics for details of contact points).
*Sold in pairs (hardware included).