Southend had been hit by a double Barnsley blast in the first half and found themselves 2-0 down at the break.
But they showed plenty of character to pull one back through midfielder Mark Gower and then haul themselves level thanks to Eastwood's piece of brilliance.
Play-off contenders Barnsley, boosted by their 3-0 win at Oldham two days earlier, had started all guns blazing and a purposeful run from winger Martin Devaney certainly raised the atmosphere from a 10,000 plus crowd.
At it was Barnsley who made the breakthrough in the 18th minute, albeit in bizarre circumstances.
Goalkeeper Nick Colgan delivered a long clearance that found the head of Southend defender Adam Barrett and the ball then struck defensive sidekick Lewis Hunt and rolled past the keeper for an own goal. Barnsley remained when the ball and both strikers Tommy Wright and Daniel Nardiello went close.
Referee Phil Josoin infuriated the Southend management and players in the first half - but then proceeded to do the same thing to Barnsley in the second half.
But for one contentious decision just before the break Barnsley went in 2-0 up.
Wright challenged goalkeeper Darryl Flahavan and the referee waved play on and when the ball came back into the box Devaney's header flew in off Kevin Maher.
Daniel Nardiello could have made it 3-0 six minutes into the second period but after a fantastic solo run he sent his shot over the top.
And that signaled the start of the Southend fight back, two minutes later Shaun Goater 's miss-hit shot fell kindly to Gower who side-footed home and then in the 62nd minute Eastwood leveled things up with a 22-yard rocket.
Barnsley thought they had snatched a winner 16 minutes from time when Chris Shuker 's low cross was turned in by defender Southend defender Adam Barrett but the flag was raised for offside.