FYI, my issue turned out to be a bad cable...switched the patch cable and was able to achieve 1000mbps.
I thought it might be an issue with the Dell iDRAC card on this Dell R610 server sharing the same network port, since the iDRAC has a max port speed of 100mbps, but I eliminated that as the source of the issue by switching this iDRAC to use a dedicated port (this was an iDRAC enterprise card).