First off you are going to have to know what rear end gears that you have. There is a tag on the rear end housing that will tell you if you know how to read it. Other than that there is a sticker on the door panel that will have the code, then you can go here to find out what it is.
If you have 3:55's then the RPM's are high but if you have 4:30's then they are about right.
As for the CEL 99% of the auto part stores can not read the codes from a Ford diesel with the code readers that they have. They have to have one that will read Ford propriety codes.