you would need to have it running at least long enough to put it in 4x4 because it needs the vacuum to engage the front axles.... but once it's in you should be ok to have it shut off

 

mine does this to i replaced the drive shaft and no change i also put it on a lift wile running and it wouldn't do it till i had to brake torque it put a load on it then it started poping trans shop i was at said it was coming from transfer case but I'm broke so it still pops lol
here is a video i made of it turn up your sound real loud to hear
http://www.youtube.com/watch?v=nz4iozlgNBg

 

With an open differential, it will only spin the opposite tire the opposite direction without it running.

Shake the front driveshaft and see if you can notice any slight play.

 

do a search.. replace your from U-joints.. should be around $300ish at the loacl shop.. of you can do it for less than $100 on your own with proper tools and Haynes manual get the loan-a-tool from local APS(auto parts store) to remove the caps

you'll have to pull the front axle as well..fyi

 

^ not axle u-joints...

if it only makes this noise in 4wd and not in 2wd, then its NOT going to be axle u-joints, since those turn all the time in both 2wd and 4wd.

the only new motions that come into play in 4wd is the front drive shaft and power through the ring/pinion, and the coupling of the CAD.

 

>The front axle u-joints were just replaced.