Neil Warnock's high flyers are just one step away from the Premiership after an extra time goal from substitute Paul Peschisolido booked a showdown against Wolves at Cardiff on May 26.
The little 31 year old Canadian international hit the target seven minutes after stepping off the bench and in an astonishing finish defenders Des Walker and Robert Page both scored own goals.
The United boss goes into his fifth play-off final defending a 100 per cent record with the Bramall Lane side who have stumbled in the Worthington and FA Cup semi-finals this season, odds on favourites to climb back to the big time.
United started brightly, midfielder Michael Tonge carved out a half chance in the 10th minute after cutting inside from the left flank. His low cross was pushed away by Darren Ward diving to his right and Dean Windass, from the tightest of angles, stabbed the ball wide.
Forest were in trouble again minutes later, a Tonge corner picking out centre back Robert Page whose header was snatched from under the bar by Ward.
United squandered a chance to snatch the lead in the 20th minute after Page touched on a cross from the left and Windass, steaming in at the far post powered his downward header straight at the Forest keeper from six yards.
Michael Brown almost grabbed his 22nd of the season minutes later, his stinging drive from 25 yards flashing a fraction wide of the target before Forest snatched the lead in the 30th minute.
Full back John Curtis tried to cut out a through ball but squared it straight to danger man David Johnson who finished clinically with a low drive from just inside the area for his 29th of the season.
Carl Asaba saw his spectacular half volley skim over the top before Forest wasted the chance to snatch a second.
Centre back Phil Jagielka tried to head clear but touched the ball to Marlon Harewood who dragged his shot wide of the far post from 16 yards.
Steve Kabba, on for Windass at half time, made an instant impact, his shot minutes after the restart stopped on the line by Ward before Forest grabbed a second in the 58th minute. Riccardo Scimeca's quickly taken free kick was fed to full back Mathieu Louis-Jean and his cross was volleyed home in spectacular fashion by Andy Reid.
Battling United were back in the game two minutes later after Des Walker brought down Asaba, Brown's free kick taking a deflection and curling past the stranded Ward .
Kabba then levelled matters in the 68th minute with an eye-catching volley.
Ndlovu blazed over the bar six minutes into extra time, Sheffield born sub Jack Lester fired wide for Forest, and Johnson's curling shot in the 109th minute was deflected before United went 3-2 ahead in the 112th minute A long ball downfield from Kenny found substitute Peschisolido who twisted past his marker, before steering the balls past Ward from 15 yards.
In an astonishing finish Walker headed a Kabba cross past his own keeper in the 117th and a minute later Page put the ball in his own net after a Darren Huckerby effort had cannoned off Kenny's chest.
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