Can I use the ball joint spacers with a long travel IFS lift kit?
Yes, long travel IFS lifts work fine with the ball joint spacers, at least those kits that use ball joints that retain the stock mounting holes. There are long travel kits that replace the upper ball joint with a higher travel uniball joint and that does not use the stock ball joint mounting holes and those of course will not work with the ball joint spacers. Some folks choose to run the spacers to allow the torsion bars to be backed off for a softer ride while retaining the same lift. Or you can adjust the bars to get increased lift.
