You should already have 4.10s, which is the lowest gearing available from the factory in our axles. You could go lower, but with only 31s, you should be alright.
AALs can weaken quick, if you buy an AAL kit, which will fight the natural curve of the leaves.
What some members have done, including Pete M, our fearless leader, is get another set of MJ main leaves, cut the eyes off, and stick them in between your main and second leaves. These are the same natural curve, and shouldn't give you any problems.
Shackles are a bad idea on an MJ. Our shackles are already fairly long, and a 2 inch lift shackle will be about 4.5" longer than the ones you have now. IIRC, the added leverage on the leaf will actually cause them to fail faster than an AAL.