Spring over trucks, especially Tacoma's, have limited bump travel. Going spring under moves the leaf spring from the top of the axle to the underside which increases bump travel. Generally, this service includes removing factory mounts/tabs on frame and axle, installing new hangers, welding on new tabs, installing bedcage, cycling the suspension to check clearance, and setting straps. That's a base spring under install but based on what you want out of the truck we can discuss to figure out what setup is best for you. Since these builds are a little more in depth, please give us a call if you have any questions or would like to discuss your build.