I had that situation come up once. I have a "no sublet" clause in leases, so I told the two who wanted to remain I had no problems with them finding someone compatible to replace roommate that wanted to leave, but that I wanted to have final approval by getting a completed App + credit report. Then I'd terminate their lease in writing (which let the departing roommate off the hook) and rewrite a new lease with the 3 new occupants. It all worked out in the end and everyone was happy.

The path you take depends a lot on your lease language regarding subletting and how your LL wants it handled. I'd suggest you speak to your LL first and see what their requirements are. Tho it's important to consider remaining roommates, it's absolutely in your best interest to keep LL in the loop. For example if your lease has a no sublet clause, and you go ahead and do it anyway, without proper paperwork and signatures, New Roommate is not party to a lease contract and technically you still are. So if a situation arises where rents are in arrears or damage is done, you could still be held accountable as well and LL has no legal recourse with New Roommate because they aren't named on the contract. (If you can get something in writing from your LL, such as a written Early Termination agreement or Release from your lease, that would be a good thing to have also.)

It is an interesting question of contract law. I'm not a laywer, but by reasoning, I would guess that you are in a four way contract with your landlord and two roommates, and therefore you would need the agreement of all the parties involved to withdraw from the contract and be replaced by someone else.

Regardless, this is a bad idea all the way around. Imagine the reverse. You come home one day and a complete stranger is now your roommate. How would you feel?

Whoever you find is going to want to meet your roomnates to make sure he/she can live with them. I can't imagine going into a shared living situation without knowing the other people at least a little, not if I'm signing a lease.

You are going to have to work the lease out with the landlord.

Since you and your roommates dislike each other so much, I doubt they will stand in the way of you getting out of the lease. And they probably won't throw roadblocks in your way as regards to the new roommate--I would say that you will have to get their approval, but I don't think they will make things difficult for you. In fact, if you are lucky, they may want to take over the recruitment of the new roommate, so that they can find someone they like.