I had one of those GMC trucks and the tow haul mode changed its shifting according to the weight. If i pressed that with an empty bed, it drove normally, add weight, it drove in higher gears but would still go into OD on very gradual down hills. THe ECT power just holds gears for longer, good for getting on short highway ramps, and using more fuel.
OD off is for towing and heavy loads that cause gear hunting on hills. Some people say OD is for highway and not city use, but thats up to you really.

Sold Truck: 89 DLX extra cab 4x4 22re Automatic/ New truck 97 T100 4x4 auto Sr5