2000 Tahoe Power Steering When Going Slowly?

[cmfnc]

I have a 2000 Tahoe Z71, old body style, with four wheel drive. When I am going really slowly, like parking or going through the drive through I get this grinding noise and it becomes very difficult to steer. I have checked the power steering fluid and the level is fine. Any ideas as to what it is? Thank you in advance.

[SKYrado03]

Are you in 4by when this happens?

Have you checked your tire pressure lately?