Not sure what you mean by drop in kits, but if it's just bulbs and the associated ballasts and wiring,
using the existing halogen projector housings, then that's probably not going to work properly. I think a lot of people install these, then stand back and look at them in their driveway, and they look a whole heck of a lot brighter. And they are brighter. But beam pattern and focal length (even if the cut off is OK), are often not as good as the engineers designed the halogen setup that was just replaced. But don't take my word for it - check out this article by a well known automotive lighting expert, who cites numerous technical documents and DOT regulations.
http://www.danielsternlighting.com/tech/bulbs/Hid/conversions/conversions.html
If a "drop in" kit involves an HID-specific projector housing, then it should work, barring any electrical problems like loads on the wiring or radio interference. See the Sticky post above in this very forum
http://www.explorerforum.com/forums/showthread.php?t=412384 for a good explanation and confirmation about how the Explorer's halogen projector housing is different from the HID housing.