1999 Passat 2.8 overheating on highway

I have a 1999 Passat 2.8, it overheats on the highway only, and does not overheat in city driving, I noticed that the aux fan does not come on when the car over heats but does come on when the A/C is on. I had people tell me that it has a cracked head gasket (no traces of water in oil), and have had people tell me that its the water pump. These are two very different items and costly as well, any help would be greatly appreciated.