Lifetime drive train warranty....THANK YOU !

I have a '99 gmc with a 4.8 and 373 gears and just returned from Jersey to Indiana towing my 2 ton Hi-Lo. I drove with it in Drive and tow mode. What I observed was that while in tow mode it never got out of third gear going up or down those mountains. I did noticed that when I started to level out on flatter roads on the western side of Ohio that the truck would shift to fourth gear at 62mph. When in the mountains I maintained a speed of 55-60mph. After I found out that fourth gear and better mmpg was at 62mph I set the cruise to 63 and enjoyed the rest of the trip home and saved a whole lot of gas at that speed. I didn't enjoy traveling those mountains one bit, and I know the poor old truck didn't like it either. I think I'll keep my travels with this setup to the central states for now. I do travel to W.Va a couple times a year and that is plenty mountains for me. So again I must ask. Why are we on purpose putting the shift lever in 3 when the truck will stay in 3rd on it's own? Are we trying to go 70mph in the mountains in 3rd,and wonder why the transmission overheats and blows up. Something doesn't make sense here in the logic.