[fongo]

you may run lean when your MAF, ACT, ECT & O2 sensors are bad. For instance, if they are dirty, coated, or old, the conductivity thru these sensors will decrease and give a misleading or faulty signal to the ECC. The ECC in return readjusts the air/fuel mixture ratio and tells the injectors and TB to deliver gas and air in undesirable ratios.
...bottom line is, with the electronics underneath the 5.0 hood, your best bet is to check the sensors or even scan the ECC and look for codes to see which sensors are bad....my 2 cents...good luck!