Your group is going to have hell of a time without a defender. That is just the way it is.
Your Wizard is doing the right thing. Leather armor proficiency, staff of defense and raising his intelligence. Magical armor will help as well. That is about the best you can do. You are not suppose to be going toe to toe with creatures on a regular basis.
If you switch out to Pally you lose your controller. Controllers are very important against minions and battlefield control.
Your problem is you have 4 strikers. Your tactics have to be hit and run if that is the make up of your group as all 4 have low AC and hit points. It wouldn't be too bad except your cleric is probably the slowest of them all. She either does not participate in the hit or is left behind.
The rangers are squishy--which is why a ranged ranger is usually better than the others. They both should have good DX and be around AC 17--likewise for the Rogue. Without the defender, you have no way of drawing attacks and allowing the rogue to setup for flanking.
The cleric is now serving as the defender /leader. If she continues that route, she should get plate armor proficiency with heavy shield.
Your best bet is for one player to take on a defender role. There is in Martial Power a fighter build called Tempest Fighter which uses 2 weapons (however they can only wear at best chain armor). I only mention it because it would if one of the rangers is a 2 weapon ranger to simulate the abilities. Otherwise, you will need to go to PHB 2 to find other defender classes outside of Paladin and Fighter.