So there is a crack in the rear diff cover from rust/rott. My question is Short of taking it off and bring it with me to the parts store, what cover is it? i think the choices are 8.5"and 9.5". Truck is a 98 tahoe, LT 5.7 4dr 4wd. I am assuming its the 8.5, but is that 8.5 internal dia...