Ford: Transmission/Towing, ford manual, materal

Question
I have a 2001 F-150 Supercrew 4.6L V8 automatic 4x2.  The transmission started slipping when towing my 5500 lb. camper.  I have a 4-way hitch/stabilizer, but I've towed less than a dozen times.  I had Aamco check it out and they say it needs rebuilt at a cost of $1195.  The truck has 70k miles on it, but that's mostly HWY as I commute 40 miles each way to work.  Is it reasonable to expect the transmission would already be shot?  Do you view Aamco as reputable?  I just went there cause they were closest.  Lastly, when towing, the Ford manual says to leave on the OverDrive, is that really better as it seems to encourage extra shifting?

Answer
the first thing i would do is a complete trans severce .have them drop the pan & inspect it for large clutch materal if you  just see some small shiny  particle this is normal wear .change the filter and make shure they replace with marcon 5 fluid if not you have more problems.also have them drain the converter witch holds about 5 to 8 qts of fluid.rbs