The home side opened the scoring through Paul Gallagher’s 19th-minute free-kick but Diego De Girolamo equalised for the Blades midway through the second half to force a replay at Bramall Lane.
Nigel Clough’s side reached the semi-final in this competition last year and could reach Wembley again if they can overturn Tottenham’s slender lead in the Capital One Cup semi-final second leg on Wednesday.
Clough had made eight changes to the side that lost 1-0 at White Hart Lane in the first leg but even fresh legs seemed to struggle
Preston began the brighter and Gallagher, who scored twice to help defeat Norwich in the last round, opened the scoring when he fired home a brilliant 30-yard free-kick.
North End then had a great chance to double their advantage on 40 minutes as Kyel Reid’s cross from the left picked out Neil Kilkenny who opted to knock across goal rather than shoot from six yards.
United almost levelled after 53 minutes but De Girolamo saw his effort cleared off the line by Calum Woods.
But Clough’s side did find the equaliser on 68 minutes as substitute Jamie Murphy cut back for De Girolamo to strike home left-footed from eight yards.