would there be any reason NOT to use 75W-140 INSTEAD OF 75W-90 in the front/rear diff? I'll be using the truck for some towing, some heavy driving in -10 to 100 degree wether...

It's not recommended for the front diff, but is optional for the rear. I had some on hand and ran it in my rear diff with no problem for about 30k miles multi-season and not towing. It came out really clean and there wasn't much "paste" on the drain plug magnet, so I think the diff appreciated it. I'm not sure if the extra viscosity would cause problems in the front, but I can vouch for no problem in the rear.