One more thing. If I get the car, I have to drive it approximately 80 miles back home. Will the rear diff be o.k.?

How can I check it's fluid level/condition and where do I put more in if needed? Tools needed?

If the car doesn't have to be on jacks to inspect/change the rear diff fluid, then I was going to drive to a nice flat spot on the way home and do the maint before I drive it 80 miles.

Also, it seems that Redline fluids are recommended a lot, what would I put in the rear diff?

Would an AutoZone or O'reilly carry this?

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1994 C280 204k miles