You have accepted the challenge of a very rugged trail. Things are going as planned when you hit a large obstacle. The steering wheel is jerked violently left then right as the truck is heading off your carefully picked line. You have bumpsteer! 

This phenomenon is a result of modifying your tie rod and drag link geometry. The tie rod and drag link as installed at the factory are very close to parallel. Keeping these components parallel reduces bumpsteer. Installing longer shackles, lifted springs, or a spring over axle lift will change your tie rod and drag link angle.

Stock Samurai Steering

Installing an after market suspension to your truck has some trade off. On the plus side you gain room for larger tires and more axle articulation. On the negative side you can upset factory caster and pinion angles.

Caster angle is established in the axle design by tilting the top of the kingpin toward the rear and the bottom of the kingpin forward so that an imaginary line through the center of the kingpins would strike the ground at a point ahead of  the point of contact. This imaginary center line is the kingpin axis Most vehicles have positive caster.
The purpose of caster is to provide steering stability which will keep the front wheels in the straight ahead position and also assist in straightening up the wheels after a turn.
Pinion angle and caster are changed when you install longer shackles, lifted springs or a improperly completed spring over axle lift.  Move the pinion up and you have negative caster, move the pinion down and you have positive caster.  This end result is due to the design of a solid axle and the steering knuckle being attached to the ends of the axle tube. As you push the pinion up or down to change pinion angle the kingpin axis moves the same amount changing your caster.

Toe In Adjustment

To run those gnarly Thornbirds you will need three inches or less back spacing. Most aftermarket  wheels I have come across seem to have a 3.5 inches measurement for back spacing. For optimum clearance most  Samurai owners  are running 2 3/4 inch back spacing. This will allow sufficient clearance  to clear the lugs on the springs during full lock turns. But it can have an adverse affect on your axle bearings. Backspacing affects your bearings because the center of the contact patch is offset from the kingpin axis. This induces torque to the knuckles.

Wheel Back Spacing

One of two ways it could be measured, based upon the definition given:
1) angle of the axles to each other, as viewed down the center of the two axles. This would give a "flex angle".
2) ramp travel index. Drive up a 20 degree ramp until one wheel comes off the ground and measure how far up you went, divide travel over wheelbase at rest and you have a ratio. If you go up without lifting a tire, you hit 1,000. a) now that you have 1,000 RTI, drive up the ramp with the trailing wheel/tire off the ramp, again until you lift a tire, measure and divide again. This will give a ratio over 1,000.
b) alternatively, set your ramp at 23 degrees and repeat the test
in the main paragraph above. This will give you a ratio below 1,000 RTI, but for 23 degrees.

An example: I ran up the 23 degree ramp to a score of 869, and up beside the 20 degree ramp to a score of 1373. Of course, when you move a ramp from place to place and set it on different surfaces, all the old figures are now off, since you can't get it back the same, unless you do it in the same spot of the shop parking lot. Remember that weight loading can affect RTI dramatically, so you have to test with the same weight (gas, tools, water, people, etc) each time.

Both have downsides/disadvantages, and neither reflects real world utility. As an example, if you had a super flexy set of $2k springs from El Specialty Spring Company, but had a stock front bumper and a double tube rear bumper, you could drive up the ramp fine and achieve a marvelous score, but when you tried to climb a rock face, the bumper ends would hit and stop forward motion. Great ramp, but no utility.