One problem related to catalytic convertor has been reported for the 1999 Ford Explorer. The most recently reported issues are listed below.
Rear windshield wiper motor failed. Caused loss of visibility through back window. Rear power window motor failed. Accelerator pedal vibrates on acceleration. (usually while merging on to highways) parking brake indicator light does not always light when parking brake is active. Rattle and vibration from catalytic converter cover.