Cheapest is to get Comcast to replace your remote with the 1067BC4, where the 4 is the important number, since it represents a new series of remotes which have the necessary setup codes.  That's probably not going to be easy, so another possibility is the RCA RCRP05B.   Here's an Amazon listing for <$10.  Use setup code Cable 1982 for your RNG cable box, and Audio 1293, or 2197, or Video (DVD or DVR) 1906 for the LG.

Other remotes made by UEI which have both the 1982 and the LG setup codes would also work.

Of course, the Harmony line of remotes will work.