Can I put JK spring on my 98? How much lift will I get?

The amount of lift you get depends on what JK springs you have. If you have springs from a 4 door rubicon hard top the part numbers will end in a 19 for the front springs and a 60 for the rear springs. Those coils are worth doing. If your spring part numbers end in a 17 or smaller for the front and a 58 or smaller for the rear, those will net no lift and you might end up shorter.