mine pops in 4wl - everyone said cv's but after bringing it to two different driveline specialty shops they both said it was the transfer case - same as you at first i thought it was coming from the front but after listening outside the car it was coming from the middle or back end - really hoping yours isnt the same as mine - i did change the diff fluid and grease the rear drive shaft before i had it looked at just to make sure those were not the problem