[dogger]

I have a first gen Tundra and like you, I tow a trailer that averages about 3500
lbs when loaded. I specified the towing package when I bought my truck which
got me a rudimentry tranny cooler and stouter electricals.

I spent a lot of time transiting between NorCal and SoCal at 70 mph and
I could rarely turn on the overdrive. I had to upgrade the brakes, suspension
and engine to better deal with the towing demands and finally, many $$$s
later, my Tundra is a decent tow vehicle.

The transmission is one of the Tundra's weak spots and has to be coddled
to make it last. Conversely, if you strengthen the tranny (shift kit, etc.)
you run the risk of grenading the weak rear end, so just go with it.

I power flushed the stock fluid and replaced it with Red Line synthetic and
added a B&M tranny cooler with it's own dedicated electric fan (B&M 70297).
This helps with the extra heat from towing but is not a total fix. Like others
have said, keep the overdrive switch off while towing. You just have to put
up with the noise and lousy gas mileage.

dogger