My 01 Sport had this, but it showed up on banks 1 and 2. I sold the vehicle before resolving the issue completely, but it can be one of 2 common causes.
1) O2 Sensor failure on the 2nd Stage O2 Sensors (rear O2 as it's considered)
2) Intake gasket failure
The O2 Sensor is easier to change, but a sensor runs you $50 easily. In most explorers, there are 3 sensors. In Explorers with dual exhaust, there are 4 sensors.
The other is the Intake gasket failure. The gaskets run between $12 and $22, but you have to remove the Throttle body and Intake to do it, which isn't really that difficult with the exception perhaps of 2 bolts against the firewall. Once you get those out, the top set of gaskets is easy, Its the lower set of gaskets I didn't Try and probably should have.
Of course that may not be all of what's causing the issue, It could well be something different, but since Your idle isn't being affected, it seems like a start. With mine the Idle Air Controller failed as well.