More info here.... Yotatech.com

.........Got a first generation 4Runner or second gen 4x4 pickup with saggy rear springs? The ZUK spring mod may be for you! It's easier than add-a-leaf springs, and the ride is WAY better. Click on ZUK's website, then scroll down to the lower left of the page, and you'll see write-ups on this mod:

GEARINSTALLS.COM

You'll see two springs suggested, one is a 12" 125# spring, and the other is a 14" 100# spring, both available from Southwest Speed, Ft. Smith, Arkansas, an ebay seller.

12" springs

14" springs

I decided to install the 12" springs to restore ride height, not add lift.

First, measuring between the edge of the wheel and the wheel well opening showed 10".

These rear springs were completely flat, and were lying against the overload leaf.

The rubber bumpers were only 1" from the stops. Every little bump in the road caused the rubber bumpers to bottom out, causing a spine-jarring ride.

First thing I did was to support the rig by the frame.

Then I cut out the bottom of the upper bump stop. I used a cutting torch because it's easiest for me. A Sawzall would work, too. Notice that the forward part of the bumpstop [on the right in the picture] is not parallel to the rear part, and will need to be bent forward to allow the spring to fit in.

I used the torch to heat up the metal to make it easier to bend.

A big hammer easily bends the front part of the bumpstop when it's red hot.

The spring fits nicely now.

I used an old spring leaf to pry the coil spring up to fit over the rubber bumper, using the brake drum as a fulcrum. You could use spring compressors as well.

With the weight of the vehicle now supported by the floor jack under the axle, notice the restored space between the leaf spring pack and the overload leaf. This is probably close to original.

A final measurement now shows the distance between the edge of the wheel and the underside of the wheelwell has increased from 10" to 13".