A plain metal rectangular block placed in between the axle housing and leaf springs. This will enable lifting properties upto an extent that your current u-bolt lengths. Basically the cheapest way to lift a leaf sprung suspension. But not the best way when it comes to flexing and cargo holding.

Shackle lifts may offer ample additional leaf sprung exntesion but the downside is cargo holding when compressed. Springs are not fully utilized when doing this kind of lifting process. It may allow you to load extra cargo with the extended lift from shackles, but be wary about the primary or the madre spring to carry all the burden load.