I'm probably missing some important detail of the relationship between the PC and the Xantech URC2 (since I have no clue what a URC2 is).

What hardware and software did you use for capturing the IR signal on the PC? What form does it store the signal in on the PC? (I might be able to figure out what went wrong by examining the data it stored on the PC for the learned signals).

The Motorola Digital Cable boxes that many of us have from Comcast use the General Instrument's standard remote control protocol. I assume the only thing distinguishing them from G.I cable boxes is the brand name printed on them. So if you have one branded G.I. rather than Motorola, it does seem like it would be the same box. If you have working IR signals for the Motorola, then I surprised (as I assume you were) that those signals don't work.