When in 2wd, the front is discoed through the t-case. Your '90 is like having manual locking hubs, but it is vacume controled when put in 4wd. hubs, and t-case are discoed at the same time. I guess jeep figured this out in '91. Only thing I see is if you are spooled in front, then the disco 30 would spin free in 2wd.