Alan Pardew's side stay in the bottom three after the 1-1 draw at The Valley, while the Blades remain two points above the Londoners with the battle to stay up likely to go to the final match of the season.
The Valley erupted after 59 minutes when a speculative long-range 35-yard shot from Charlton's Moroccan defender Talal El Karkouri somehow managed to beat United goalkeeper Paddy Kenny and finish up in the back of the net.
But the much-maligned Jonathan Stead produced an equally stunning equaliser on 69 minutes to give the Yorkshire side the point which steers them slightly nearer to safety.
United arrived having lost eight successive away matches and having failed to score in their last four.
But they were composed and resilient and, after taking an early battering when Charlton failed to take advantage of their superiority, Neil Warnock's side deserved a share of the spoils.
Charlton had three good chances early on with Darren Ambrose heading wide from a perfect Dennis Rommedahl cross, Darren Bent firing wide from an Ambrose corner and El Karkouri seeing Kenny tip over his goalbound free-kick.
Warnock immediately changed his system to play just one up front and United could have snatched the lead when Keith Gillespie's header appeared goalbound before goalkeeper Scott Carson produced a late save to turn the ball away for a corner.
Charlton took the lead out of the blue when there appeared no danger as El Karkouri unleashed his 35-yard shot which bamboozled the United defence and left Kenny floundering.
It looked like the goal which would give Charlton the precious three points, but some scruffy play from them in midfield let in United and Stead allowed the ball to roll into his path before hitting a beautiful right-footed shot into the bottom corner.
Charlton were getting more and more edgy and United even sensed victory as the home side threw substitute defender Hermann Hreidarsson up front.
But United defended stoutly and the home side had to be satisfied with their second successive draw at home.
With only two points out of the last nine they are clearly feeling the pressure, while United, with two home games to come against fellow relegation strugglers Watford and Wigan, look to have done enough to stay up.