wxmn6,

The two sets of numbers refer to the thickness or viscosity of the oils. Very similar to motor oil like 15W-40.

The 75W-90 weight gear oil is what comes in your truck differential and is recommended most of the time.

The 75W-140 is listed in your operators manual as an alternative gear oil if your tow heavy loads in hot weather. Heavy loads would be 12,000 lbs. or above. Several people on this site feel that even when towing heavy, the 75W-90 will still be fine.

When towing 14,000 lbs. I use the 75W-140.

I hope this helps.
Don