Another thing to look into is take your truck out on the highway but keep od locked out. Get a good solid 55mph, turn od back on. Notice the shift and watch where the rpm go to. Now set the cruise and watch for any further settling.
What you're watching for is called torque converter lockup. P1740 often gets thrown when a converter has worn to the point it'll no longer lockup. I myself ran mine for 5years like that. Downsides are large heat build, greater wear and lower fuel mileage.
You don't HAVE to set the cruise but I've noticed on multiple trucks that getting it up to a constant speed doesn't necessarily always trigger lockup if the trans is still a little cold. Cruise triggers it 100% of the time though.