This answer was provided by William Severance on the dnn community exchange site.

Mark,

It's actually much simpler than you're making it . . . Assuming that your ajax request is already setting the ModuleId and TabId in it's beforeSend callback using the service framework's plugin function setModuleHeaders, you will find that within your custom service controller's methods the ActiveModule property (ModuleInfo object) is already set for you. ActiveModule exposes both the module's ModuleSettings and TabModuleSettings hash tables. So, you should be able to write:

var URL = ActiveModule.ModuleSettings["SuccessURL"].ToString();

Note that unlike the Settings hash table exposed by PortalModuleBase or ModuleSettingsBase which combines ModuleSettings and TabModuleSettings to create the Settings hashtable the ActiveModule property exposes settings as two separate hash tables.