A late header from Chris Porter sent Sheffield United into the League One play-off final at Stevenage's expense.
Porter came closest to opening the scoring in a drab first period, only for Boro goalkeeper Chris Day to pull off a fine save.
The visitors improved after the break and Luke Freeman and Scott Laird forced home keeper Steve Simonsen into action.
But, with time running out, Porter won it with a far post header from Matt Lowton's cross.
The Blades, relegated from the second tier last season, will now return to Wembley for the first time since losing in the 2009 Championship play-off final.
Returning to Bramall Lane after the goalless first leg, the hosts almost got off to the perfect start, but Porter's close-range header was spectacularly saved by Day.
That early chance was a rare bright spot in an otherwise attritional first period, with both teams seemingly putting more emphasis on defence than opening the scoring.
The two lone front men, Porter for the hosts and Patrick Agyemang for the visitors, were too often isolated against resilient defences with both sets of midfielders struggling to find space and creativity in a congested middle.
As per the first leg, it was Luke Freeman who looked most likely to force the issue for Boro, but he too cut a remote and frustrated figure by the interval.
Both managers will have been grateful for the sound of the half-time whistle, particularly visiting boss Gary Smith, whose side failed to register a single attempt on Simonsen's goal.
The second half was a distinct improvement on the first and United went close three times in quick succession.
First, Ryan Flynn's shot from inside the area was blocked four yards out, then Porter's cross was fired wide by Lee Williamson at the near post before Flynn lashed a half-volley just wide.
This roused Boro, and Freeman linked up well with Scott Laird before embarking on a mazy run that culminated in his side's first shot of the night, easily saved by Simonsen.
Stevenage began to sense they could cause an upset and another Freeman run ended with a super save by Simonsen from a well-struck Laird effort before Michael Bostwick fired a free-kick straight at the keeper from 30 yards out.
With home fans becoming restless, both Matt Hill and Stephen Quinn went close before Porter settled the contest and United nerves with a towering header at the far post from Lowton's excellent centre.
Boro sub Chris Beardsley forced a save from Simonsen with a header from 18 yards but their dream of three successive promotions came to an end with Danny Wilson's side holding out to spark scenes of wild jubilation as home fans rushed on to the pitch at the final whistle.
The Blades will face either Huddersfield or MK Dons in the final on Saturday, 26 May.
VIEWS FROM THE DRESSING ROOM
Sheffield United manager Danny Wilson told BBC Radio Sheffield:
"(I feel) very proud, but we were pushed to the limit tonight.
"Although we've played very well I think some of the football we've played has been outstanding. We had a real game plan and the lads carried it through to the letter.
"And thankfully we've come up with a great goal at the end to win it."