+1 on the front axle disconnect. we've got 6"+ on the roads here right now, so 4wd is pretty much mandatory. mine has almost the exact same symptoms..I'll add it does it more when cold than hot ( fast idle=reduced vacuum=disconnect), more when going uphill than down ( reduced engine vacuum=axle disconnect) and 'pop' is a very realistic recreation of the noise....does the 4wd indicator go on and off as well? more prone to doing it if you leave the t-case in 4wd overnight? ( mine QUIT doing it when I switched to 2wd and disconnected the axle at night...my guess on that is condensation / frost plugging the lines, or the vacuum pod?!?!)

check the front axle acuator, check the switch on the t-case, check the vacuum reservoir behind the front bumper, and check all the hoses. a vacuum gun ( eg: mighty vac) is really helpful, as you cant generally hear the vacuum leaks with the motor running.