My LTD carrier contracted with an advocate co. to put me in for SSDI. They are filling out all the forms for me - it might be worth looking into because they only collect if you receive disability. And their max fee is set by the SSA.
Here's a link - sorta like reading the income tax instructions
Disability is based on being able to do any work. Anyways, the younger you are or the more education you have, the more adaptable you are considered to be and therefore more able to do skilled or unskilled sedentary work. My case worker told me that once you hit 55, you're basically considered to be untrainable. So either you can continue to work at something very similar to what you have been or you are disabled.
As I recall, if you are still working and making more than like 860/ month, you are not eligible. Think you have to have been out of work for 6 months and will expect to be out of work for at least a year.
It's all on the SSA web site, it's just not fun reading,