Can I use longer shocks in the rear with this lift shackles? If so what length do I need? And will anything else need to be done ?

You should be able to get longer shocks, especially if you get monotube shocks that compress enough to allow full stuffage too...and with your SL, the tires still stuff up to the same point as they did w/o a SL.

The shocks for the lift kits should be the right length, or you can go custom.