I've measured my wire diameter and it's at 15.4mm. From the picture I posted above, one side has 6.5 coils and the other has 5.5 coils.
With these numbers, I've run the calculation to find the spring rate as they sit in my truck according to the equation:
k=(dG)/(8NC^3), where:
N=number of active coils=3.75 on driver side
D=mean coil diameter=6.4in
d=wire diameter=0.6063in
C=D/d
G=modulus of rigidity=11,600ksi
I come up with 200 lbs/in. HOWEVER, these springs are compressed to the point where the coils are touching each other at the ends, thus the small number of active coils used in the equation. The effect of this is the spring rate is artificially higher.
To back up the theoretical side of this, I also did some testing on my 4runner. The wheelbase is 102in and the distance from the rear wheel to my rear bumper is 42in. With 150lbs of me standing on the bumper, the bumper sags approx 1in. Doing some basic statics and free body diagram, I find the force the rear springs feel is around 211lbs and the deflection at the springs is 0.7in. That makes the spring rate 211/0.7=300lbs/in...but wait, there are two springs in parallel so that means each individual spring has a spring rate of 150 lbs/in.
So the theoretical comes out to 200lbs/in and the experimental is 150lbs/in. What does this mean? Well, unfortunately, a small error in measuring can make a big difference in the result so it's difficult to say how accurate the experimental result is. My conclusion is that the spring rate of the stock LX springs are somewhere below 200lbs/in and above 150lbs/in. I know, a huge difference, but it's a start anyway. I can take better measurements the next time I see my friend, it's kinda hard to measure things by myself.
Some of the spring rates for OME rear coils ranged from 220-260 lbs/in (I think 260 was the high?), per a past thread, so I'm in the ballpark. Please feel free to check my work above, that's why I posted it.
So, here's another comparison. This is a picture of coils from an earlier 80:
http://community.webshots.com/photo/103759693/117565896PiWeOL
Compared to the ones I posted above, the early 80 coils have almost 7 coils, while mine have 5.5 and 6.5 coils. Also, the wire diameter appears to be thicker, but it's hard to be sure. This adds to the confirmations from toothfairy and beowolf.
Based on Dan's part number post above, I conclude that 96-97 FZJ80's and LXs have a softer spring, while the FJ80s and FZJ80s before 96 have stiffer ones. This contradicts some posts in the link in the 80 series FAQ.