"Not only are you wrong, but I even created an Excel spreadsheet to show you how wrong you are." --James Wyatt, May 2006
Dilige, et quod vis fac
If you are locking down a bunch of enemies adjecent to you, it sounds like you are doing the job well. More than any other role, defender is a job. You have to get out there and make the enemies attack you rather than the squishier party members. I think I would shy away from the berserker because you can leave you party defenderless if you rage, and while they will probably survive fine, even short combats without a defender can be a real drain on surges. Knight is a solid class, but can be a bid monotonous compared to a full AEDU class.
If the warden is working, I think I would look toward feats as others suggested. Warden are defenitely a high hp/surge class, probably the best as those go. There is something appealing to me about having a character that can take a huge beating and shrug it off when other character would be crying for the medic or making death saves.
I dont like to take any of the rage dailies since they last the whole encounter and I dont want to be in my fury the whole encounter.