explorer 1997 transfer case

been having repeated problems with clunking noise in the rear end. Took
vehicle in several times, dealer could not find anything wrong. The
vehicle then would go out of gear while driving, then the dealer said the
problem was in the transfer case. $1800.00 later & a month later the
clunking noise has returned. The dealer says he doesn't know what the
problem is, but has to take the transfer case apart again & doesn't know
what the cost will. How many times is required to fix this problem?????