Back in 1995 I purchased a 1990 Chevy 1/2 ton short box pickup (350 2 wheel drive) for my son. I had to put a new transmission in many years ago at my chevy dealer and they told me if I was going to tow my 2000 lb boat, I should shift down a gear so as not to be towing in drive mode. We still subscribe to that advice no matter the vehicle. In my v6 Camry I shift down to 5th gear when towing anything such as a rowboat or uhaul trailer. Is that sound advice to protect the transmission.
your 90's pickup probably had 4 gears. The idea of towing in 3rd was to prevent torque converter slip (which creates heat), and to increase engine RPM which keeps the transmission fluid pump speed high to move the fluid faster (dissipates heat). Later with the electronically controlled transmissions, GM implemented a "tow/haul mode" switch which basically accomplished the same thing.
I'm told that modern transmissions have so many speeds (6, 8, and more) that they can lock up the torque converter much earlier, which translates to less fluid shear and lower fluid temps. So low, that newer vehicles often omit the transmission cooler.
Just stay within your vehicle towing limits, follow the severe service interval for fluid change, and any other guidelines given in your owner's manual.
