That 4-2 win also ended Shrewsbury's two year spell in League One.
Jon Taylor headed Shrewsbury in front but Conor Washington levelled soon after with a shot from inside the box.
Washington fired past Shrews keeper Chris Weale to make it 2-1 before Lloyd Isgrove added a third from close range.
Tom Bradshaw's strike reduced the deficit but, after Britt Assombalonga missed a penalty for Posh, Josh McQuoid nudged in a fourth for a comfortable win. In the end Swindon's 2-0 defeat at Notts County made their task impossible and along with resurgent Sheffield United they miss out.
Peterborough join, Preston, Orient and Rotherham in the play-off battle for a place in the Championship. Peterborough have already tasted Wemble success this season by lift the JPT last month.
At the foot of the table with Stevenage also relegated last week, there are still two teams set for the drop. carlisle are now deep in the mire after a home defeat to Oldham who are now safe. Tranmere are 21st, a point behind Crewe and two behind Notts County. Because of their goal difference County only need a point from their last game at Oldham.
Crewe entertain Preston and Tranmere host Bradford on the final day.