It is fairly common with the 4.6 to have a hanging idle. They were programmed with quite a bit of airflow between shifts and even when at idle while moving. While it is common, there are a couple things you can do to tame it. Some IAC valves are worse about a high idle than others. If you aren't using a motorcraft IAC, replacing the IAC with a motorcraft/factory valve can help. You can also program around the issue using a tuner or having someone that knows what they are doing tune your car. The typical functions with an effect on the IAC are related to the idle dashpot and there are multiple functions and values that can be changed to affect the performance. Another option is the IAC mod. this is basically a modification which places a restrictor inline with the IAC valve. This reduces the maximum airflow the IAC can move, and thus reduces the hanging idle issue.