The Blades looked threatening at the start and home goalkeeper Gary Woods did well to save an 11th-minute free-kick from defender Tony McMahon before Doncaster took charge.
Rovers striker Billy Paynter gave them the lead from the penalty spot in the 18th minute when he slammed the ball home after visiting goalkeeper George Long had brought down David Syers.
The game was one-sided for long periods after that with Doncaster defenders Jamie McCombe and Rob Jones going close before striker Iain Hume hit the post early in the second half.
Syers made it 2-0 in the 63rd minute with a close-range header from winger David Cotterill's free-kick on the left.
The Blades had been disappointing until then, but Nick Blackman gave them a lifeline they hardly deserved when he scored from the penalty spot in the 80th minute after Andy Griffin had brought down Blades defender Joe Ironside.
Rovers finished with 10 men after Paynter was sent off for a foul on McMahon off the ball in the 87th minute and United cashed in with a last-gasp equaliser when Dave Kitson fired home in a crowded penalty box after visiting defender Matthew Hill had hit the post.