Does it only do it just after you accel from a stopped position?
most likely...but not definitely, the yoke on the front of the driveshaft is hanging up on the output shaft for the Xfer case. There is a new designed yoke available that will correct it...its nickel plated.
But if you want a cheaper and easier way to permanently fix it...pull the driveshaft and put a super ball in side the yoke and reinstall the shaft. Sometimes 2 super balls are required, but it will work and will eliminate the klunk.