Lionard Pajoy’s cross set up Boskovic’s header, and United (15-10-5) earned their third straight win and extended the club’s record home unbeaten streak to 15 games.
United moved into a tie for third place in the Eastern Conference, but the New York Red Bulls still hold the first tiebreaker of total goals scored.
Chivas (7-15-7), last in the Western Conference, lost its season-high fifth consecutive game.
United, which dominated early, regained control after halftime and took the lead when Pajoy and Boskovic connected. Pajoy got the ball on the right and curled a cross in toward the near corner of the 6-yard box.
Boskovic, a substitute, made a darting run from near the penalty spot toward the ball, dove in front of a defender and redirected his header inside the left post and beyond Dan Kennedy.
Boskovic, who also has five assists in 22 appearances—including 11 as a sub—signed with D.C. as a designated player in June 2010 before re-signing a less expensive contract this June.
The midfielder could play an expanded role down the stretch with the season-ending injury to captain Dwayne De Rosario sustained earlier this month.