3.21 gear trucks are extremely sensitive to changes in drag or inclination, i.e. wind and hills. I have been in mind unloaded traveling a now stretch of road with a constant head wind (no sure what, but higher than normally) and couldn't keep the truck in 8th gear even at 72-75mph when she normally sits in 8th.
I was swapping 6/7 nearly constant above 75 and it only settled down to a 7th with an 8th gear hit every so often when I dropped down around 70. It felt like I was dragging a 6000lb brick behind me. Again I was unloaded, not trailer, nothing, and I know the section of road and how she normally drives. I had had other trucks thru here with stiff winds in the past but never noticed it as much due to lower rear gears, 3.73, and a lower top gear, i.e. 6 speed tranny.
The size of 8th gear and the tall rear gears make these trucks "hunt" between 7 & 8 as they are only running 1800 or so at almost 80 mph. Therefore any additional load an the tranny shifts down.
I love my truck but really would have like to have tried on 3.55s as I think they would have been a good mix for me and slowed the hunting for most of my driving. But as others will tell you, if you tow often you want 3.92.
Now the nice part of these transmissions, is they are so damn intuitive. I would never let someone else, particularly a 21 year old with no trailering or heavy payload driving experience, drive any of my past trucks / vans on the highway towing any real load, even on fairly flat road (i.e. Buffalo to Albany NY), never mind a 97 Sonoma on a full car trailer. However, I had enough confidence in the transmissions capacity to predict, select and shift appropriately even under those conditions I put my son under the wheel for around an hour or so of our 4 hour trip.
While she may drink fuel, she will tow like a dream in my opinion. So Just fill her up sit back set the cruise around 68-72 and just let her do the work.
Side note, when towing you may want to step up to 89 octane in the tank, as the engine was designed to provide its best / better performance on 89 octane and its possible you could see 1 to 2 mile per gallon more, i.e. at 3.68 per gallon and 9 miles to the gallon your cost is 0.41 cents per mile, at 4.11 per gallon for 89 octane and 11 miles per gallon you would only spend 0.37 cent. Not a huge amount, but $1 cents on every 25 gallons, or 50 additional miles on every 25 gallons, i.e. 225 mile on a 25 gallon tank or 275 miles on a 25 gallon tank and that's actually a $9.69 less to travel 275 miles on 89 octane at 4.11/gallon because you only use 25 gallons, vs 30.5 gallons at 9 miles per gallon to travel 275 miles..............
How many miles is your trip each way?
And is that the Displayed MPG on the dash, or hand calculated? I find my dash is 0.5mpg to almost 1mpg high depending on driving done, when hand calculated.
So if its off by 0.5mpg (i.e. 8.5 on 87 octane & 10.5 on 89 octane), then a 300 mile trip is about $12.50 less on 89 octane...... Just think that's a free 6 pack of beer at camp