There are a lot of things that you could do before spending more money. How much clearance do you have? Could you pull a leaf out of the front to better match the rear? What about a small lift block? Or, as you suggested, longer shackles. I would be weary of getting them TOO long though, lots of extra leverage and in my experience they tend to bend the spring eye if they are excessive.

Harry Wagner
Harry Situations
4x4Wire's Toyota Section