Assuming this is a procomp type IFS lift you may have a minor problem with the half shaft bolts hitting the truck frame on the left side. To see if this is your problem put the truck in 2wd, and unlock the hubs. Now crawl under and turn the half shafts by hand. If the bolts holding the shaft onto the diff is hitting then this is your problem. To fix it loosen the three bolts holding the third member in place, then use a long shaft to push the third member back as far as you can. Now just tighten up the bolts and your done.