In their loss to the Colts Sunday, the 49ers gave up 179 yards rushing.The stat is the clearest indication that this Niners defense isn't what it has been over the past two seasons.As Chase Stuart of the New York Times noted this week, the Jim Harbaugh-led 49ers had only given up 170 yards or more on the ground once in his first 38 games (including the postseason) as San Francisco's head coach. The 49ers have now allowed more than 170 yards rushing in consecutive games.Is it just a blip? Or is it a trend?In 2012, the 49ers were fourth in the NFL, giving up just 94.2 yards on the ground per game, an average of 3.7 yards per carry. In 2011, they ranked No. 1, giving up just 77.2 yards per game and 3.5 per carry. This season they rank 28th, giving up 138 yards per game and 3.9 yards a try, including six touchdown runs -- just one less than they allowed in 16 games a year ago.As the 49ers (1-2) head into their divisional game against the Rams (1-2) in St. Louis Thursday night, the defensive fall from grace by San Francisco is even more alarming than the team's struggles on offense.And the 49ers go into the game without starting outside linebacker Aldon Smith and perhaps without...