Castillo broke a scoreless deadlock when he broke open on the left side and launched a high shot past RSL goalkeeper Josh Saunders for his first of the season.
Jackson assisted on the tally and then doubled the advantage in the 81st minute when he made Saunders pay after a bad first touch, putting the ball into the goal off the netminder's feet.
FCD (3-1-0) grabbed sole possession of second place in the West with nine points, one behind the Los Angeles Galaxy, which beat Colorado Saturday.
Salt Lake (1-2-1) remained winless in 12 all-time visits to Dallas.
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