What are the benifits of a rear spacer, vs. AAL when lifting the rear end? Havn't managed to figure this out as I try to educate myself.

A spacer aka block will raise the rear without changing the spring rate. It will also contribute to axle wrap.

An additional leaf will raise the rear a little, both by the thickness of the leaf and the minor increase in arch provided by that leaf. A single leaf will not bring the back up 2"...a single leaf will be lucky to add much more than its own thickness to the rear height. The single leaf, and additional leaves, will increase the load handling ability. They will not change the weight rating, only the truck's ability to handle a load. Multiple leaves will raise the rear by their own thickness and of course by any additional arch they provide to the main leaf.

If you do not change the main leaf, you have not changed the amount the spring can deflect, only the amount of weight required to make it deflect a certain amount.

thanks- makes sense