Sone Aluko grabbed a double and Steven Whittaker, Jamie Ness and Alejandro Bedoya claimed a goal apiece as the Glasgow giants ended any hope of Motherwell beating them to the runners-up spot.
However, with United now unable to catch the Steelmen in third place, the result at Ibrox also hands Motherwell a crack at the Champions League qualifiers next season, with administration-hit Rangers ineligible for a UEFA licence.
Rangers made two changes from the side who lost to Celtic in Sunday's Old Firm derby for the clash with United.
Salim Kerkar and Ness were drafted into the starting line-up, with Lee McCulloch and Andy Little both rated doubtful beforehand and only McCulloch making the bench.
United were unchanged from the side who drew with Hearts for the visit to Glasgow.
Ibrox legend Sandy Jardine addressed the crowd before kick-off, telling them: "Now is not the time for division. Now is not the time for protest and disruption. It's time to get behind both bidders."
His comments came amid rumours of a protest at tonight's game against American bidder Bill Miller, while rival bidder Brian Kennedy was believed to be attending the match.
Once the action was underway, Rangers were ahead after just six minutes when Rhys McCabe teed up the shot for Whittaker and he produced a lovely finish to slot past Dusan Pernis and into the bottom corner.
United also looked dangerous and Allan McGregor was called into action to tip a Gary Mackay-Steven effort just over the crossbar at full-stretch.
Rangers doubled their lead after 16 minutes when Garry Kenneth lost possession to Kerkar and he played the ball to Aluko, who did well to race past the United defence before firing home.
Aluko was celebrating again four minutes later when he burst into the box again and rifled low into the back of the net, leaving Pernis with no chance.
Rangers were hungry for more goals and Whittaker unleashed an audacious shot from 35 yards that fizzed just over.
Aluko then supplied a cross from the left for Kerkar but he was just unable to connect at full-stretch in front of goal.
The home side continued to pile on the pressure and Pernis blocked from Lee Wallace, before claiming a Kerkar header on what was proving to be a busy night for the United number one.
United made a double switch at the interval, with Barry Douglas and Keith Watson thrown into the action for Mackay-Steven and Kenneth.
Aluko could have added to his tally for the night when he collected from Kyle Bartley only to see his effort palmed onto the side-netting by Pernis.
Instead, the fourth goal was claimed by Ness who picked up the ball on the edge of the area, amid Rangers claims for a penalty for hand ball, and drove home after 57 minutes.
The midfielder, who was back in the side after overcoming a number of injury problems, was withdrawn a few minutes later for Kirk Broadfoot.
There was to be no hat-trick for Aluko, who left the pitch to rapturous applause after 74 minutes, allowing Bedoya to make a rare appearance.
McCulloch - who was an injury doubt ahead of this match - was then given the last 10 minutes in place of Kerkar, who had also impressed for the Light Blues.
It was Bedoya who put the gloss on the win when he raced onto a pass from fellow American Maurice Edu and fired low past the United goalkeeper with six minutes to go.