This is a few reason s why we are talking about the rear leaf springs ,Coil springs and SINGLE leaf spring give a Consistent spring rate, Its harder to get a Consistent spring rate with a leaf spring pack, there is a certain amount of lost motion, as the axle moves in bump and rebound, and engages and disengages with the secondary leaves. There is also friction between the leaves . also when the car is accelerating, the spring will be trying to wind up, causing axle tramp. Lowering blocks that are too tall create more leverage on the spring and increase spring wind up. So if you are running a live rear axle, the ideal is a 4 link suspension with coils springs, but a leaf spring rear end can be nearly as good, If can get your rear leaf springs working correctly.

I agree with Harry first get some new tyres . and have a look and how the old tyres wore on the tread face .