4x4 Sub front wheel noise

I have a 2000 suburban, and when i activate the 4x4 Hi it engages no problem. I notice when i come to a stop or slow down and turn there is a whining noise from the drivers front wheel. The rear wheels will actually slip trying to move the front wheels in a turn, till they straighten out and get up to speed again. I was wondering what this problem could be, and if it was serious? I seldom use the the 4x4, but will activate it to keep the fluid and parts lubed. please help.