Excuse me, but have you been able to isolate whether it is really a problem with the password string, or with your sas account, or with the site you are trying to access with proc http?
If it is a problem with the special symbols in the password, are you double coating the password when specifying it in proc http? Why don't you use single quotation marks?
Use single quotation marks when you don't want to expand or reference a macro.