I have a 2000 Expedition 4x4, 5.4 L. The 4x4 is the AWD, 4hi, 4lo type. When it is in a load (such as stuck in snow) and in 4lo/hi and you give it much throttle it will make a slipping, well a buzzing/grinding sound from the transfer case. When this happens the 4x4 is not working or the tires are not turning on the front any way. Also this will happen if you really hit the throttle from a dead stop in AWD mode. I had a mechanic friend tell me that it could be the vacuum actuator not working properly or even the fork that it operates may be bent. I have looked around and have not found any one discussing similar problems. Any ideas on this one?