North End, now unbeaten in 13 matches, move up to sixth but would have had two more points if Tyrone Mears had converted a glorious late chance.
As it was, United, whose ambition was limited throughout a low quality 90 minutes, never really looked threatened but Reading could open up a six-point gap if they can beat bottom-club Millwall tomorrow.
It was a high-tempo start but the first decent opening came on six minutes when United defender Rob Kozluk gave the ball away with a slack cross-field pass, but David Nugent shot straight at United goalkeeper Paddy Kenny after racing onto Adam Nowland's through-ball.
Blades midfielder Phil Jagielka blasted well over from a complicated free-kick routine and the action continued with Youl Mawene heading wide from Graham Alexander's free-kick at the other end. With Nugent a constant threat, Mears had a header blocked and David Jones saw a curling free-kick well held by Kenny.
North End left-back Matt Hill latched onto another mis-placed pass but Nugent, who was booked for tussling with Kenny kick, shot wildly off-target.
As the half-hour approached it was the visitors' turn to have a spell of pressure but when they were able to conjure an opening, North End goalkeeper Carlo Nash got a hand to Danny Webber's low shot and turned it around the post.
Jagielka had a shot saved by Nash after Webber's swift break and Mears saw a low effort pushed over by Kenny at the other end but the score was goalless at the break.
The home side poured forward from the first whistle of the second half but there was little to show for their early pressure apart from a Nugent shot which sailed high and wide after Danny Dichio's knock-down.
After Jagielka's persistence sent Webber into the box, the midfielder again failed to make the most of a good opportunity, blasting over when the ball was deflected back to him.
Following a long spell of mediocrity, North End finally put a good passing move together but Kenny blocked Chris Sedgwick's low cross as once again quality possession came to nothing.
Neither side was showing any fluency, and Nugent and substitute Lewis Neal both failed to work Kenny with more hopeful strikes from outside the area.
Dichio glanced wide from Alexander's cross as North End continued to plug away but the chance of the match fell to Mears, who skied from six yards from Dichio's knock-down with eight minutes remaining.
United had a half-hearted appeal for a penalty for Nowland's challenge on Jagielka turned down and Lee Unsworth struck a late free-kick wide and the game ended in stalemate.