4 Wheel Drive/SUVs: Improving traction, bf goodrich, yokohama geolandar

Question
I own a Nissan Patrol 260 2.8 Diesel, 1991.
The vehicle is stock except for the shocks (EMU Nitrocharge)+ wheels (yokohama geolandar A/T 205/80R16).
The vehicle has 90 HP.
Stock size Wheels are 205R16.

I live in Spain and I use the car for mountain trails and African trails.

In sand dunes I experience problems and often get stuck.
The engine has limited HP and I don't want to touch it.
So the solution would be improving suspension and tires.

1. Would taller springs help?
2. What tire size would you recommend considering that in trails I do not only d dunes, but some rock crawling, normal trails and quite a bit of asphalt to get there! A friend gave me his set of 15" wheels and 31x10,5R15 Marshall A/T tires, would they do? Would you recommend a brand (I was thinking BF Goodrich A/T KO
3. Would ARB rear lockers help me in sand dunes?



Another question:

My leaf springs are sagging in front. Does it significantly reduce my off road capabilities?
Can a shop correct them? Is this kind of repair reliable?

In advance I thank you for your suggestions

Answer
Carlo,

Let's start with tire size.  The 31's should do just fine.  If you are having trouble in the sand, my first suggestion is to lower your tire's air pressure.  In the sand, you should not be running more than 15 psi, and you can go as low as 8 psi.  If your rims are 8" wide, than you can run a little lower pressure than, if they are 10" wide.  I wouldn't suggest going to a larger tire, simply due to the fact that you are limited on power.  

As far as the lockers  go, they are always a big help, whether in the front, or the rear, especially in the rocks.  If you do very much rock crawling, you might consider installing some lower gears in the transfer case for a lower low range, such as a 4:1 kit.

Weak leaf springs, do not, alway reduce your off road capability.  Sometimes it even helps, as they will allow for more articulation than stiff springs.  You don't want them to get to the point that they are ready to break though.  You can take the springs to a reputable blacksmith, and he can re-arch the springs to stock height, or he can add a little more arch to give you a small amount of lift.

Just like any hobby, the more extreme you want to get, the more modifications you need to do to your vehicle.

Carl