Nothing was really "threatening" about the Constitution of 1963. As early as the 50s the U.S. labeled any reformist (like Arbenz in Guatemala) a "communist" regardless of the State's class relations. The '63 Constitution was relatively liberal, but surely not a threat to U.S. economic interest. The reason it received the reply it did was because the U.S. didn't want "another Cuba" and after decades of supporting and backing right-wing dictatorships in the Dominican Republic a introduction to a liberal democracy was considered a leftist move.

A minimal agrarian reform was considered communist back in those days.

Leonel is, like you mentioned a puppet of the U.S. So of course, a lot of the PLD's essential characteristics are discarded since it is now a complete capitalist party. Matter of fact, some leading PLDeistas claim that Bosch and Balaguer were the "same" ! . The same people that aided the overthrow of Bosch are endorsing the PLD now. People like Elias Wessin who were the subordinates of Trujillo and were put in power by the U.S. after the Coup D' Etat.