Its almost certainly the oil cooler. Its a common problem and very easy to change on the 2.8 whereas head gasket failure is pretty much unheard of.
Make sure you flush the cooling system
thoroughly after changing the cooler as the emulsion gets everywhere and hangs around. I have a V8 which suffered the same failure, and after a new cooler was fitted it was written off simply because the cooling system wasn't cleaned properly so there was still oil in the coolant
I used Forte Bio Degreaser to clean it out which was very effective.
