The Blades had earler led before going 2-1 and 3-2 behind before John Egan's 94th-minute equaliser forced a replay at Bramall Lane.
United’s Daniel Jebbison was sent off as the Blades finished with 10 men, and here’s how we rated the United side in Wales...
5. John Egan 6
Many home fans were baying for a red when he upended Mullin as he looked to race through on Davies' goal but a covering defender probably saved the Irishman's skin. Then popped up with the equaliser at the other end with a great finish to force a replay Photo: Gary Oakley
6. Chris Basham 6
A professional display from the veteran who seemed to always be in the right place at the right time, apart from one hairy moment when the ball bounced off him and into the path of Mullin who blazed over Photo: Gary Oakley
7. Ben Osborn 4
Started at wing-back and ended in midfield before making way. Couldn't really get into the game apart from a few dangerous forays down the left early doors Photo: Gary Oakley
8. Ollie Norwood 7
Brilliant ball over the top sent Jebbison clear but Howard was quick off his line to snuff out the danger and prevent the Blades doubling their lead before the break. Then a composed finish dragged the Blades level at 2-2 after falling behind Photo: Gary Oakley