That sounds weird that your ZJ was basically level (I presume), then by swapping the springs the rear is now lower. I've weighed my 4.0 ZJ at work and with a full tank of gas plus 100# in the rear, the front came in at 2,220 pounds, the rear was 2,050, and total weight was 4,360. (I know the totals don't add up.) I would expect that springs designed for the front would be stiffer to handle the constant extra weight. But then, maybe since the front weight is relatively constant, maybe the rears are a little stiffer to handle the occasional extra loads. I don't know.
Is your front end higher after the spring swap? Or does it ride stiffer?
I don't remember how much lift the ZJ V8 front springs would give the rear, but I'm sure it wasn't 3.5 inches. I think it was somewhere between 1.5" and 2". Why don't you just reinstall your springs in the original locations?
'96 ZJ, 4.0L, 42RE, NP242, stock D30 and D35-LS w/ 3.73, 4-1/2" lift, 32" BFG ATs, Magnaflow, JKS track bars and disconnects, spare tire carrier, Optima red top, ROLA roof rack, KC SlimLite driving lights, CB and Yaesu ham radios, GPS