Yes, CONSTANT backshifting should be avoided. Anytime a tranny consistently upshifts and downshifts - such as when towing up long grades, one should shift into a lower gear (or out of OD). This is a normal procedure in any vehicle in order to keep the trans happy. My towing monster (02 f350 chipped diesel) was no different. I would take it out of OD and ocassionally into a lower gear, to avoid excessive downshifts and upshifts....however, it would still pass just about anything on those grades pulling 9,000 lbs.

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Cheers!
Chris

03 X5
02 Acura
88 Mustang GT Ragtop
88 Campion Allante 195
99 Itasca 29