If it is in collections, it has likely been sold to a company that recovers debts. Therefore it is unlikely that you will be able to work with Washington Mutual to settle.
Of course they want more, but if they are willing to work with you they can take what you can give, or they can get nothing (unless they pursue court). Be firm but honest with them, and don't for a minute think they are working in your best interests.
I would not send them post-dated checks. There is no law that says they have to wait until the date of the check to run it through, and most banks have it in their policy that they will payout on any instrument presented to them without regard for the date.
If they are hounding you on the phone, you can tell them you would like to be contacted only through the mail and say "
do not contact me again via phone to collect this debt." If you tell them they cannot call you, the have to stop or they are in violation of the Fair Debt Collection Practices Act.
You find some great information here:
Fair Debt Collection Practices Act - You can Fight Back!