Weird clack from front end, help

I have noticed on my 500 that when it is in a load, or under acceleration in first or second gear, there is a pop from the front, the more on the gas, the louder it is, i have tracked it down to the front shaft going to the diff., when u jack the front of the bike off the ground and put it into 4wd. when i spin the front tires they will spin, then suddenly clank and come to a stop, take it out of 4wd, they wont do it, put it back into 4wd and after a few times it will do it again, is something messed up that would keep it from not fully ingaging and disingaging out of 4wd? when u are riding it it sounds fine, no problems in 3rd or up. But when u take off it sounds like it slips, then pops and u are fine after that.. thanks chris