Tranmere moved five points clear while closest challengers United and Stevenage both lost.
Adam McGurk volleyed home Jake Jervis's cross for the only goal at Boundary Park as Ronnie Moore's men registered their 11th win of an increasingly impressive season.
The Blades saw their unbeaten start to the season come to an end at MK Dons for whom Shaun Williams scored a last-minute penalty and Stevenage were thrashed 4-1 at home by Preston.
Stuart Beavon and Greg Tansey traded early goals before Jack King gave the visitors the lead just after the half-hour mark.
Jeffrey Monakana and Nicky Wroe then completed the scoring in the second half as North End moved to within three points of the play-off spots.
Crawley, in fourth, were held to a 1-1 draw at fifth-placed Notts County.
Former York duo Andre Boucaud and Neal Bishop combined for the latter to put County ahead after 12 minutes but Jonathan Forte's tap-in on the stroke of half-time earned his side a share of the spoils.
Swindon climbed into the top six after a 2-0 win at Walsall.
Paolo Di Canio's side took a first-minute lead through Simon Ferry and Matt Ritchie made the points safe in the second half.
Brentford moved up to eighth but had goalkeeper Simon Moore to thank as they beat Carlisle 2-1.
Moore saved Joe Garner's late spot-kick to maintain his side's five-game unbeaten run.
Jonathan Douglas hammered home a far-post volley to put Brentford ahead but Carlisle levelled when Garner cleverly made space for himself to fire past Moore.
Paul Hayes grabbed the winner shortly after the hour mark.
Bournemouth made it seven games without defeat as Harry Arter scored in their 1-0 win at Doncaster while on-loan Joe Edwards netted the winner as Yeovil beat rock-bottom Hartlepool by the same score.
Pools remain six points adrift of second-bottom Shrewsbury, who were beaten 2-1 at Leyton Orient.
Jon Taylor cancelled out Kevin Lisbie's 33rd-minute penalty but former Charlton striker Lisbie headed in an 88th-minute winner.
Adam Lockwood and Steven Schumacher scored as Bury beat managerless Portsmouth 2-0 at Gigg Lane and Scunthorpe's resurgence under returning boss Brian Laws continued at Coventry.
Fresh from their stunning 4-1 win at Walsall in midweek in Laws' first match back in his third spell in charge, the Iron won 2-1 at the Ricoh Arena.
Carl Baker put the home side ahead in the 36th minute but two Leon Clarke goals, the first from the penalty spot, won it for Scunthorpe, who are now just a point behind the 20th-placed Sky Blues.
Colchester squandered a 2-0 lead to go down 3-2 at Crewe.
Freddie Sears and Ian Henderson put the visitors in command, but Crewe struck six minutes into the second half through Lauri Dalla Valle.
Luke Murphy converted a 64th-minute penalty and Mathias Pogba headed home the winner 10 minutes later.
