120Y main leafs are longer. They are not interchangeable.

the number one cause of axle tramp is shagged shocks. if that's not the problem, just get another standard set of leafs from a wrecker, disassemble, cut the main leaf at the rear edge of where it sits under the diff and add it upside down to the front half of the top of your leaf pack. You'll also need to cut some off the front so it sits just behind the eye, but not touching. And round off the underside of those cut edges so they don't abrade the main leaf as they slide along it as the suspension travels.