I would have thought 75w/80 is more of a gearbox oil as it is slightly thinner than 90 - but as my dad said when his sump cracked and leaked oil everywhere, he temp repaired and filled with what ever he could get hold of - mum being mum, she said that oil he bought wasn't the right one, he said it was all he could get hold of, and wrong oil is better than no oil....
Although 75w/80 is listed in some cases as a Diff oil, Land Rover sepcify 75/90, so maybe when you a spare 10 minutes, to drain and refill.