Simple version of what jjvande said:
the radius arm itself does not carry the weight of the vehicle unless the arm is the only thing touching the ground, that is the job of the spring. theoretically, if the XJ was dropped straight down, the arm wouldsee very little force on it.
The arm does, however, carry the weight of the axle, so if you were were trying to figure out if the arm whill bend if you drop it on a rock, then just use +- half the weight of the axle (driver/passenger side will not be equal because of the pumpkin) times the distance from the axle connection end (because of torque)
If the arm is what is holding the vehicle up, the weight on it would be determinate of how many other tires were on the ground.
if you were trying to figure the force on the arm when the tire hits the ground it becomes a little more complicated. If the arm itself does not contact a surface, then only the component of force along the arm will have any effect on it. I am assuming that since you are doing stuctural testing that you know how to calculate that.
the above is a "spherical horse in a vacuum" approach to finding forces but for what you need i think it will work. :greensmok