You have the 231 T-case. Like the others said, only use this on slippy surfaces. The case "locks" the front and rear driveshafts together to turn at the same time, so once in a while a tire needs to slip a little bit to not mess up the case. (That's what I was told anyway). 99% of the offroading I do is done in 4lo. The only thing I would see 4hi being used for is sand. HTH
'87 YJ, built 351W, T-18, NP-205, SOA, SRS, 35x12.50 ProComp MT's, cage, winch, Tomken bumper/carrier, d44/9"