Seems to me that diff
oil coolers are used on roundy-round cars all the time for the same reason. There is no heat radiating surfaces on the diff, because under normal use, they don't get that hot.
There are little trans
oil coolers that come with electric fans that can be mounted in any position. Couple this with an electric
oil pump, and a little plumbing and it should drop your diff oil temp.
Last time I visited the coatings shop, they were coating diffs with heat dissipating material on a race car, along with trans cases, oil pans, and brake rotors & calipers.