Have you confirmed that the front drive shaft isn't turning when in 4wd? I'd jack up the rear end, engage 4wd and see what happens just to be sure. This test should help confirm where the problem is. I think it could be possible you might still have a problem in the front axle with a stripped gear in one of the front final drives which may "appear" to be working unde little load but then slip under real loads. The drive pinion in the tranny may also have issues, or the 4wd lever could be slipping on the shaft.