Trundle will be labelled the hero of the game at Vetch Field, but it was the 55th minute arrival of substitute Andy Robinson that sparked the home side into life.
Within 11 minutes of replacing Kevin McLeod, Robinson broke the deadlock with a low shot in off the far post and suddenly there was far more urgency in the Swansea approach.
Trundle pounced with two goals in the last eight minutes to shoot his side back into an automatic promotion spot.
Both sides took time to settle, and it was midway through the opening period before the first opportunity came to the Swans.
McLeod opened up the visitors down the left flank, but his pin-point cross was headed over the bar by Trundle.
Swansea started taking control and Sam Ricketts made several darting runs down the right but his crosses failed to pick out the waiting Lee Thorpe and Trundle.
Cambridge rarely threatened the Swansea rearguard, but in one isolated breakaway, veteran former Wales international Iwan Roberts forced a save from home keeper Willy Gueret.
Swansea were almost caught cold in the opening minutes of the second half, when Gueret failed to hold a shot, and Tes Bramble went in for the loose ball only to see Kevin Austin thrust out a foot to clear behind for a fruitless corner.
Robinson was brought on 10 minutes after the restart in an effort to pep up the punchless Swans, and he was in the right spot to find the net after Trundle had worked the ball across the area.
The visitors used all three substitutes in an effort to rescue a point, but it was leading scorer Trundle who finished them off when he raced on to a Robinson through ball before coolly beating the advancing John Ruddy in the Cambridge goal.
Trundle was far from finished and with two minutes remaining, he raced through the middle before whipping a low shot into the corner of the net beyond a motionless Ruddy, for his 22nd league goal of the season.