I've heard Padre Island National Seashore can have great runs of jacks near the beach and Mansfield jetties. I'm not sure when is the prime time but Billy Sandifer's best times chart shows late spring/early summer and October and November. Check out his and Eric Ozlins articles on Texas Saltwater Magazine website archives around that time, I think there are some mentions of jacks. I have a buddy that wants to do this with fly rod, that would be awesome. Jacks are a great fighting fish, good luck to you.
Many men go fishing all of their lives without knowing that it is not fish they are after. Henry David Thoreau