Here are the ypc for each of the Cowboys last 4 opponents.
Houston - 3.1/ypc
Indy - 3.5/ypc
NYG - 4.3/ypc
Minnestoa - 4.3/ypc
Those aren't exactly numbers that will blow your doors off. In fact, the league average right now is 4.5 yards per rush, so the last 4 weeks Dallas has held good rushing attacks to below the league average. Holding a team to sub-4/ypc is an excellent week. I think this notion that the run defense is not improved is a fallacy.
I think the rushing defense has actually been much better than early in the season. I would have to look at the numbers, but it seems that they are surrendering far fewer explosive runs. LVE, Barr, and Damone Clark are flying around a lot faster and our DEs are setting a much harder edge. The problem now lies in the war of attrition going on in the defensive backfield. Kelvin Joseph and Nashon Wright are going to end up being a big issue for us if they can't figure something out. The pass rush was negated in Houston by the quick passing game. Their wideouts had DBs beaten so quickly there was no way for the rush to get home. This is what worries me more with Tom Brady, one of the best quick passing QBs of all time, being the likely first round matchup.