Joined: Mar 2008
Location: Prairie Village
To remove and replace the axle, the differential cover had to have been removed. Hence, unless for some odd and unlikely reason they reused your old oil, they would have had to put in new oil. I honestly wouldn't worry about it.
My 1996 manual says that it only needs to be replaced every 100,000 miles or if you drive through water up or over your axle.