OP my guess is you have the 4R70w transmission - Ford AOD transmission - Wikipedia, the free encyclopedia
& TCCoA Tech Articles
- biggest thing IMHO is trans temp if you don't have a cooler you should look at getting one, if the fluid is 10 years old you may want to consider getting it changed (not to close to trip time though) with regards to OD on or off the Fords towing guide state:
"Driving with an Automatic Overdrive
• With certain car and compact truck
automatic overdrive transmissions, towing
– especially in hilly areas – may result in
excessive shifting between overdrive and
the next lower gear.
• If this occurs, it is recommended that the
overdrive gear be locked out to eliminate
the condition and provide steadier
performance (see your vehicle’s Owner
Guide for information).
• When there is no excessive shifting, use the
overdrive gear for optimum fuel economy.
• Overdrive also may be locked out to
obtain engine braking on downgrades."