For the relocation to work properly the Backup folder inside %AppData%\MobileSync should be moved to drive D:\ for example into a new folder called D:\MobileSync, leaving %AppData%\MobileSync empty and more space on the C: drive than there was before. %AppData% should resolve to C:\Users\<User>\AppData\Roaming.
If you moved the backup data as described then the following command would relink the data folder to path that iTunes is expecting.
MkLink /J "%AppData%\Apple Computer\MobileSync\Backup" "D:\MobileSync\Backup"
After issuing the command the data would appear to be exactly where it was before, but the space is only taken up on D:, not on C:. When iTunes is run the existing set of previous backups should show under Edit > Preferences > Devices.
tt2