Does RedSn0w see your device either in the normal state, Recovery Mode or DFU mode?
If possible, create a new administrative user on one of your computers, or, if possible, create a new partition and install a fresh copy of OS on that partition just to see if you can resolve this issue. Install a fresh copy of iTunes (and all of it's co-dependencies and such) and see if that resolves the issue for either iTunes or RedSn0w.
Also see if connecting another USB device works on any of the computers, such as another iOS device, another phone/tablet (Android, BB, HTC, or the like), a flash drive, and any other variety of USB if you'd like. This will see whether or not this issue is solely for communication devices or is it for certain types.