Recently bought an 04 XLS-Sport 4x4. This topic is interesting to me because I thought that the AWD models were RWD until wheel-slippage was detected, where it would then transfer torque to the front wheels as well, therefore "usually" being 2wd for most uses, and thus being more MPG-economical. The AWD models are also rated, I believe, 1 or 2mpg above the 4x4 models.
Mine is a 4x4, and is in 4x4 all the time without being able to go to 2wd. Its default is AUTO 4x4 which as I understand it works similar to the AWD system in detecting wheel slippage, where it directs more torque to the wheels with the most traction. Also it has 4x4 Hi and 4x4 Lo which can be engaged and removes the slippage detection and just puts 25% torque to each wheel.
I'm probably wrong though, but that is what I thought in regards to the AWD versus 4x4 models... AWD models are 2wd (rwd) until poor conditions are detected, 4x4 are full-time 4wd.
I'm new to the Explorer and 4x4's completely, so let me know just how far off I am!
All the best,
Scott