thats because with 4wd model they expect the truck to be towing thats why the use the different gear oil. you can you use the 75/140 if you are goin to be towing with. but make sure that you use a full syn. gear oil and get some mopar limited slip fuild dont use the stuff that you can get at your local parts store.

 

But wouldn't you want a heavier rear gear oil if towing? 4WD you would think will have the 75W140 in the rear end, while the 2WD would have the lighter 75W90.
Seems backward to me.
oldhp