Back in August, I suggested that the Lo Duca's defense might be overstated by defensive Win Shares. One test would be to see if his defense remained stable from the Dodgers to the Marlins. As it happens, he had 5.3 defensive Win Shares in LA and 1.8 in Florida. That's over 779 2/3 innings verus 413, which works out to 0.00680 WS/inn. vs. 0.00436. So it would appear that Lo Duca benefited from the Dodgers good defense.
Unfortunately, this is a really small sample. It's easy to imagine a catcher slowing down over the season and becoming less effective, which is just the reputation Lo Duca has. A better study would include lots of trades with catchers moving from bad defensive teams to good and vice versa. This still might not be enough.
The other way to look at the effect of the trade would be to compare team defense before and afterwards. The data isn't really available, however. (At least not without doing a lot of work.)