Check all the parts of the shaft and the t-case and pinion flanges for play. Also check the phasing of the u-joints:
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http://www.4crawler.com/4x4/CheapTri....shtml#Phasing
And if that is all OK, try some of the tips listed below:
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http://www.4crawler.com/4x4/CheapTri...FixingProblems