I use M1 75w90 front and rear. Cheapest at AZ. The Walmarts around me don't sell it. Check your manual for proper fill height. Mine was not fill till it drips out. It was lower (5/8" below fill hole front, 5/8 - 1-5/8 below rear) If you had to choose between the two, no doubt do the front diff. I'm changing that once a year, always dirty, always kind of silver which is bad. The rear was done for the first time at 100k. Came out almost perfect.
06 Escalade 6.0L AWD 155k. M1 EP 5w30 Fram TG
03 Maxima 3.5L Unknown miles ST 10w30