i have an 87 XJ with a NP231 transfer case, recently I'm unable to shift into 4 low. The 4 HI works fine... I've tried manualy disconnnecting the linkage and im still having this problem. When i attemt to engage 4 low it makes a loud klunking noise followed by this nasty grinding sound. Im just wondering if it could be the chain slipping... Greatly appreciate it is anyone can help!!