Sensors don't typically go bad very often.
Here's the thing, though:
All emission-related components, including the cat, oxygen sensor, etc... are all covered by a Federal Emission warranty for 8-years/80,000 miles. So the Ford dealer should have replaced the cat and the O2 sensor at no charge to you.
How many miles are/were on your truck?
At the least, your mechanic should have advised you of this, if you were under 80,000 miles (unless he fixed it for free, which I doubt).
If it's warrantied thru them, though, then they should fix it for free.
Here's the text from your 2003 Emission Warranty booklet, page 12:
EMISSIONS PERFORMANCE WARRANTY COVERAGE
If your vehicle is registered in a state where the state or local government has
an EPA-approved inspection and maintenance program, you may also be eligible
for Emissions Performance Warranty Coverage for 2 years or 24,000 miles
(whichever occurs first), if you meet the conditions listed below in this section.
Under the Emissions Performance Warranty for passenger cars and light duty
trucks, Ford Motor Company also provides coverage, including labor and
diagnosis, for 8 years or 80,000 miles (whichever occurs first) for the following
emissions parts, if you meet the conditions listed below in this section.
• catalytic converter
• compuvalve
• electronic emissions control unit (PCM)
• onboard emissions diagnostic device
• vehicle communication link (VCL) module