Today i got a hell of a deal i got 5 quarts of 85w140 valvoline oil for only .50 cents for all 5. So my question is will that work in my diff of my 90 tracker 8v 5sp with 31's and stock gearing.

I'd put that in the spring/summer/fall but not in the winter. It's a little heavy but in the heat it'll be fine.

I wouldn't put it in - not even in the summer

What's the factory spec ?  I'm guessing 75W90 or similar - that 85W140 is going to be a lot heavier - try pouring it out of the bottle, you should be able to SEE the difference - 85W140 - the 85 before the W says it'll be no thicker than 85 in the cold and the 140 after the W says it's be no thinner than 140 in the heat.  It's meant for a vehicle hauling a LOT more weight than you are.