The recommended avenue for repair/replacement of any NI product is by calling our support line to set up the RMA process. You could do this by contacting our customer service at (800) 531-5066. Be sure to have the serial number and part number of your USB-6008 ready, because this information is necessary for an RMA. This process may involve some troubleshooting steps, but since you have 3 other functioning devices it sounds like it is certain there is a hardware issue with the 4th.
I did call and found something interesting. Apparently with the older (8.0) driver what was supplied with the first USB-6008, when it was set to limits of 0-5.0V output, it would actually only go from 0-4.99V. When 5.00 V was requested, the output went to zero. It was not clear initially that this applied to all units. Installing version 8.3 of the drivers appears to have solved the problem.
Thanks for the update - I was unaware of this and will make note of it. In any case, it is always recommended to install the latest release of any driver distribution. With new driver version releases, features are frequently added and issues such as this are corrected.