I have a 3.0 V6 and I lock mine out of O/D whether the trailer is empty or loaded, though once in a while on flats or gentle downslopes I'll sometimes bump it back into O/D, probably 90% of the time it's towing I have it locked out of O/D. I do the same on my 4Runner and Dodge Ram P/U as well.

--Kevin

'93 T100 4x2
26MM Torsion bars, Bilstiens and RS9000's, Firestone air springs, 2.25" Magnaflow exhaust, Hella 550 driving lights, etc.
'99 4Runner Limited 4x2 -- Hers
'87 Toyota X-Cab -- Her Kids'
'04 Silverado Crew-Cab P/U -- Toy Hauler