My parking brake is completely useless, so much so that I can pull the handle all the way and still move the truck myself without much effort. I had already tightened the adjuster under the passenger door as much as I can. When I had my brakes replaced, I told Nissan about the p-brake and they said they'd look at it. When I got it back, it did actually work "somewhat" for about 2 days, then it was useless again.

I've been told there is also an adjuster in the rear, and when I look at the line in the rear there is a lot of slack. I got under there last night, but couldn't figure out what exactly it is I need to tighten to take up that slack. I did tighten the nut thats just to the right of the diff (where the lines from the passenger side attach to the bracket thats attached to the axle housing) but that really didn't make much difference. Am I looking in the wrong spot?

Also, the manual says I should have between 10-12 clicks of the handle for full engagement.....I get about 26. Nissan said they didn't see anything wrong with it, but I really need to get this fixed. I park on a pretty good hill every day, and its a total PITA to turn the truck off every time I get out of it when I'm wheeling, otherwise it'll roll.

I've had this happen some time ago, and Mobycat just did very recently:
There is a "pulley" behind the drum that connects two lengths of cable in the parking brake mechanism. It gets frozen onto the pin that it pivots on. You pull the handle, the cable goes taut as normal, but it's pulling against the frozen pulley, not through it to the rest of the mechanism that actually sets the shoes against the drum.

Pull your drums and check to see that the pulley is free to move. If it's frozen on there, good luck getting it off without damaging it, it's cheap pot metal. Broke both of mine, one of which needed replacing, the other just had a small piece chip off and has been OK since.

Brent

Definately check out what Brent stated, it's a very good possibility that is what it is.

Also, what can happen is that the rear brakes can be out of adjustment, causing a lot of slack in the system.

If you are going to get it checked at Nissan, then make sure that they double check the rear brake adjuster for sticking. It's something that you can also do, if you're familiar with drum brakes.