Ok, so this is an odd item. Here's the situation:
1. FedEx Freight LTL Registers properly
2. Rating returns an error that says "Authentication Failed".
Work-around resolution:
1. Find the FedEx Parcel Carrier.
2. Jot down the Web Service version number.
3. Delete the Web Service version number out.
4. Re-register FedEx Freight LTL.
5. Add the FedEx Parcel Web Service version number back in.
This appears to have worked for Talley when they had this issue.