In a nutshell, rear springs and shocks need to be slightly firmer than fronts for the vehicle to have a stable response to bumps on the road. The front hits the bump first, so the rear needs to be a bit firmer to catch up.
It helps to have a test road where the bumps are evenly spaced, because that exaggerates the effect of unbalanced springs, such that you can feel the lack of balance even if your shocks are new. There's a test mile close to home on I-295, where the bumps are evenly spaced, so I got experience with balanced versus unbalanced springs. I can tell the difference between (1) factory-balanced springs/shocks (e.g., newer car), (2) close to factory-balanced springs/shocks (e.g., install Monroe/Tenneco shocks/struts, and you'll be close to factory-balanced, but not exactly), and (3) very unbalanced springs (e.g., worn out rear leaf packs at 90k miles).
When they design an OEM suspension, they decide on front firmness based on handling and comfort (e.g., "drop" testing, etc.), and then the rear is mathematically determined to be the right amount firmer than the front.
Instead of me just writing about it, here's what the AEV website says, since that's where I first learned about the concept:
". . . Since there are front and rear springs, the forward and rearward halves of the Jeep actually represent two spring-mass systems that must interact with each other. To understand the concept of frequency-based spring rates, think of a shock-less vehicle driving over a single speed-bump. When the front end hits the bump it starts to oscillate up and down at a certain speed. This is the front’s ride frequency. The rear encounters the same bump at a time delay determined by wheelbase and vehicle speed. The key is that the rear needs to react faster than the front so that the oscillations of the rear can catch up to the front in about one cycle (from ride height to some amount of ‘up’, then ‘down’, and back up to ride height). This is important because if the vehicle doesn’t naturally tend to level out quickly after a bump, the shocks will be overtaxed with trying to control body position/motion instead of their real purpose of simply getting rid of the oscillations.
"So to ensure the best possible combination of ride and handling, the front and rear spring rates must be derived to create the proper front and rear frequencies relative to one another. . . . To further enhance the spring’s ability to maintain proper frequencies under varying load conditions, a suspension engineer will design a progressive-rate spring (especially for the rear), which will keep the frequencies closer to constant over the expected load range."
I think people who tried the ZJ/ Crown Vic combo found the Crown Vic springs to be a bit too soft for the rear of a TJ that has OEM ZJ springs up front. There's probably a spring that's suitable for the rear of a TJ that's balanced with front OEM ZJ coils, but I haven't done the research/testing to know what it would be. To save time and money, I stopped looking at springs after I found my second replacement set to be balanced and otherwise satisfactory.
It's worth switching to dual-rate springs or progressive springs. You get to have a huge improvement to offroad performance without compromising ride quality. The spring rates published by Moog for my combo are more than 40% higher than rear OEM TJ springs, and more than 25% higher than front OEM TJ springs. With factory springs, I was always bouncing to full compression when offroad. With my current springs, I haven't touched the bumpstops yet, even though I'm now driving a lot faster when offroad. At ride height, my dual-rate springs have a very soft frequency, so I got best results with Bilstein 5100s. I like the combination of firm shocks and soft springs (at ride height).