There are a number of factors impacting a D's performance. To try and single out 'motivation' as the difference is pointless and just suits whatever preconceived arguments someone wants to make.
Personnel, scheme, coaching, the opponents we're facing, etc. all play into the D's performance.
Personnel--Not a huge lot of changes since the beginning of the season, but there have been a few (Jones out, Lattimore in, etc).
Scheme--DC tried putting in a new scheme at the beginning of the season, didn't work so well.
Coaching--coaches prepare the players for the schemes as well as for that week's opponents. I have no insights to how those coaches are doing their jobs, but it's possible that they're better preparing the players for the game (including motivating them).
Opponents--last two weeks are teams we know very well--their strengths and weaknesses. That familiarity helps in preparation. Vikings were also a wounded kill--minus AP and having to start Ponder at QB. That's not the steepest defensive challenge. That said, it was the first well-rounded performance by our D this year. I was satisfied with our run D for the first time this season (I was NOT impressed against the Bears--they were gashing us for 9-10 yd gains in the first half).
In any event, no one can prove their side/convince the other in a 'it was their motivation' - 'no it wasn't' argument.