Transfer case or Transmission problem? Or something different?

This may be an interesting one for you. I have an '02 Ford Explorer that has had a few transmission problems and I'd like to know what the problem is now. When I drive the car it works great for a few minutes and then while I'm driving it disengages from being in gear. When I press the gas at this point it simply just revs the engine and the rpms shoot up. After a few seconds of being disengaged I will feel a pop and then the car works fine again. It has no problem shifting when it is working properly but it pops out and in probably 3 times within a 2 mile drive. Any suggestions? I think It may be the transmission yet again but I'm thrown by the fact that it shifts perfectly without any dropping sound. One thing to note is that it drops out when I am NOT touching the gas. A few websites have suggested that it may be the transfer case. You guys know best, what do you think?