1. 2" shackles on the rear-- why would you only do this and leave the front alone? it puts more weight up front on a truck (which already has a high front weight bias). i have heard the shackles decrease spring life. is this true and why?

Most people who put shackles on had already raised the front end with their torsion bars. Most will be moving to front end lifts as they become available from manufacturers.

Also many already had weak rear spring packs and were using them to offset some of the droop, or were using them as an intermediate step before getting a new spring pack as part of their lift.

I have a replacement steel bumper which will have a tire carrier and heavy spare on it soon. I'll be using the add-a-leafs and/or shackles to keep the back fender well off my tires.

I don't know about the spring life. The Nissan spring life is so short I don't know how you could tell.