Trundle plundered the equaliser in a frantic second half spell that saw three goals in eight minutes as the two promotion rivals went hell for leather looking for a win.
Barnsley squandered a string of first half chances and were hit by an Andy Robinson goal two minutes after the restart.
But as the hectic start continued Daniel Nardiello levelled for the home side and Marc Richards took them in front from the penalty spot after being brought down by Kristan O'Leary who was then red carded.
It was left to Trundle to salvage the point for Swansea with an emphatic strike in the 64th minute.
Barnsley should have been in front after nine minutes when Richards headed on a high ball and Nardiello made a brilliant break before sliding an inch perfect ball for Martin Devaney who hoisted his first time shot over the open goal with time to control and shoot.
Swansea keeper Willy Gueret produced a pair of crucial saves to stop Barnsley in their tracks as they looked to extend their impressive home form that has seen them beaten only once at Oakwell.
He pulled off a brilliant reflex stop to turn over a volley from recalled striker Nardiello who chested down and smashed his shot from close range.
And when Nardiello hit a dipping free kick from 25 yards Gueret made a diving save at full stretch to push the ball away from the top corner.
But the visitors rocked Barnsley after a half time rocket from manager Kenny Jackett. In a high energy opening Alan Tate delivered a terrific cross from the right and Robinson nipped in to head from close range through Nick Colgan's legs as the keeper tried to scramble the ball away.
In the 56th minutes a long punt from Colgan sent in Nardiello behind the defence and he rammed home his shot after Gueret dithered.
Three minutes later O'Leary bundled over Richards in the area and the striker took the resulting penalty himself to claim his 11th of the season. O'Leary was red carded for the foul.
And in the 64th minute Trundle charged in from the right and crashed a shot from 15 yards.