IIRC, one of the easy lift mods on a ZJ was to put the front coils in the back and then get a lifted set for the front, which netted the equivalent of a budget boost. (I'm operating from memory here, so you should verify this information with a search, maybe on
www.nagca.com or over at JU.) Since ZJ front coils provide approximately a similar effect when swapped into a XJ front, I predict that putting stock XJ front coils in the back of a ZJ will not gain any lift.
Here's a less wordy explanation:
ZJ front > ZJ rear
ZJ front > XJ front
XJ front = ZJ rear
I'm sure there are different variables at play here too, including options such as engine choice and towing or upcountry suspension options, so keep in mind that my analysis is highly simplified.
But, if you want to determine the spring rates for each, compress each spring with a known weight and measure the deflection. K=F/X, from high school physics.
Or, for a rough idea as to which spring is stiffer, measure the wire diameter and the number of turns. A stiffer spring will have a thicker wire and fewer turns for the same static height.
It seems to me that the best bet for the ZJ would be just to pop in a set of coil spacers, since you are ensured of a given lift for the same amount of work.
