Preston had already booked their place in the top six and rested several key players for the trip to Pride Park.
There was certainly a low-key feel to the game but Derby went in front just before the break thanks to Inigo Idiakez's free-kick.
Derby needed only a point to qualify for the play-offs and they took another step towards that target when Tommy Smith scored their second goal four minutes into the second half.
Jeff Kenna made a darting run and, although he was tackled on the edge of the penalty area, the ball broke for Smith to fire a low shot home.
The visitors deservedly pulled a goal back in the 69th minute. A precise cross by Graham Alexander caused chaos in the Rams penalty area, and although Lee Camp made one great save, Patrick Agyemang fired the rebound into the roof of the net.
Preston should have equalised in the 81st minute when Agyemang used his strength to force his way into the area but when he pulled the ball back, Dickson Etuhu fired wide from 12 yards out.
The game was now opening up as Preston pressed for an equaliser and Derby almost scored on the break when Paul Peschisolido brought an excellent save from Carlo Nash.
But Derby restored their two-goal advantage in the last minute with another counter-attack. This time Peschisolido got past Claude Davis and calmly rolled the ball under the advancing Nash.
The result means Derby finish fourth and now will play Preston in the first leg of the semi-final at Deepdale next Sunday with the return leg the following Thursday.
It is unlikely that Preston will be an easy to beat when the teams meet again although Derby have an excellent away record to take with them into the game.
But the Lancashire club will be confident of progressing to the final after beating Derby 3-0 earlier in the season.