Scrum half Ben is one of five changes from the team that won the first Test 23-21 in Brisbane, coming off the bench to replace Mike Phillips. His Leicester Tigers clubmate Tom Croft goes the other way, changing places with Dan Lydiate in the back row, while Tommy Bowe replaces try scorer Alex Cuthbert on the right wing.
Cuthbert is retained in the matchday 23 as a replacement, taking over the role that Sean Maitland had in Brisbane on the bench, while injuries to Alex Corbisiero and Paul O'Connell have forced the Lions to make two further changes in their front five.
Mako Vunipola moves up from the bench to start at loose head prop, while Geoff Parling takes over in the second row from O'Connell. Further changes among the replacements see Ryan Grant, Sean O'Brien and Conor Murray all come into the Lions Test arena for the first time.
"We said after the first Test that we had improvement in us and we are confident that this is the case. Several players have put their hands up this week and have forced their way into the Test side while several players have returned to fitness," said Lions head coach Warren Gatland.
"That is typical of Lions tours. We want players to constantly push for selection, we then select who is on form and who is available. Based on that we have selected a squad that is capable of winning the Test and securing the series."
The Wallabies have been forced into two changes in their starting line-up through injury and have made a further two among their replacements. Robbie Deans' side will be looking to keep the series alive by winning in Melbourne on Saturday - just as they did 12 years ago to take the series to a decider in Sydney.
The Lions haven't won a series since 1997 and are now just one win away from clinching the three-match rubber and claiming the Tom Richards Trophy for the first time.
"We are one Test to nil up and have a wonderful chance to win our first series since 1997. Sixteen years is a long time to wait for a series win. We were very close in 2009 and the whole squad is firmly focused on victory this weekend," said tour manager Andy Irvine.
"The Wallabies are always a tough proposition at home and showed in 2001, after losing the 1st Test, that they are capable of coming from behind in a series. We will not underestimate them and will be prepared for a Wallabies side that has to win to keep the series alive."
British & Irish Lions (v Qantas Wallabies, 2nd Test)
15. Leigh Halfpenny (Cardiff Blues / Wales)
14. Tommy Bowe (Ulster / Ireland)
13. Brian O'Driscoll (Leinster / Ireland)
12. Jonathan Davies (Scarlets / Wales)
11. George North (Scarlets / Wales)
10. Jonathan Sexton (Leinster / Ireland)
9. Ben Youngs (Leicester Tigers / England)
1. Makovina Vunipola (Saracens / England)
2. Tom Youngs (Leicester Tigers / England)
3. Adam Jones (Ospreys / Wales)
4. Alun Wyn Jones (Ospreys / Wales)
5. Geoff Parling (Leicester Tigers / England)
6. Dan Lydiate (Dragons / Wales)
7. Sam Warburton (Cardiff Blues / Wales, captain)
8. Jamie Heaslip (Leinster / Ireland)
16. Richard Hibbard (Ospreys / Wales)
17. Ryan Grant (Glasgow Warriors / Scotland)
18. Dan Cole (Leicester Tigers / England)
19. Tom Croft (Leicester Tigers / England)
20. Sean O'Brien (Leinster / Ireland)
21. Conor Murray (Munster / Ireland)
22. Owen Farrell (Saracens / England)
23. Alex Cuthbert (Cardiff Blues / Wales)