Shackles won't affect had mileage. Did you do a slip yoke eliminator kit on your tcase when you lifted? If not, your first drive might be your last until you do put one on. I did a 2.5" lift with 5/8" boomerang shackles and had to lower my tcase an inch to make it driveable and it still has a supper light vibe at 40mph. I am going to swap my springs around though because I put them on the wrong ends of the jeep (front to rear) so I am hoping swapping eliminates the vibe. If not, I am going to have to remove the bommerangs and try out the old stock shackles and if that doesn't work I will just have to get an sye kit.
I don't feel like working on anything at this time so i sold my yj.