2.5" lift shackles? So the shackles are 12" long? Or are you saying they are 1.25" lift in which case they are only 8" long.
In any case, you're really edging up to needing an actual suspension lift of some kind. 2.5" lift with the TJ flares should get you there.
The problem I see is that your leafs are probably inverted some so they aren't even flat anymore. This means you're starting with less lift then stock and the shackles aren't helping the leafs any. Not to mention your changing the caster on the front axle by rotating it. It's also probably throwing off the pinion angle on the front and rear axles.
With a body lift that tall, you're going to have to stretch all of the fuel lines and extend the steering shaft. You will also want to extend the shifters as they are now 2" lower then they once were. At least with a new body lift, you'll be replacing all of the old bolts which are probably rusted and eaten away. Problem is you need to get them off and not break them off. Also, if your body mounts aren't in the greatest shape, the body lift could cause the sheet metal to tear.
If it were me, I'd go with a OME 2.5" suspension lift which will give you 3". Add that to the TJ flares you alreay have and you should be good. Costs more but then you're getting a better setup.
