A friend of mine is about to add a lift to his JL Rubicon and I’m wondering if his stock springs will fit my TJ. Has anyone put the springs from a JLR on a TJR ? How much lift does it give you?
Measuring the free lengths won't tell you much other than the free lengths. Since you don't know the JL spring rates or your Jeep's sprung weights (assuming), you can't know anything about the ride heights until you try it out.
Several have asked. To my knowledge, we don't know what actually happens. Early on, I read that the JL springs are unique to each corner, including the physical fit.