LTD claim denied after 2 years - insurer says I can do 'sedentary work' despite two surgical opinions

I was approved for long-term disability in 2023 after a spinal fusion surgery that left me with permanent nerve damage in my left leg. I can't sit or stand for more than 20-30 minutes without significant pain. Two different neurosurgeons have documented this. My occupation is software engineer - technically "sedentary" by dictionary definition but requires sustained sitting at a workstation for 6-8 hours a day.

My insurer just sent a denial letter saying that based on their independent medical examination, I am capable of "sedentary work" and therefore no longer qualify for benefits under the "any occupation" definition my policy switched to after 24 months. Their IME doctor spent 18 minutes with me. My own surgeons, who have been treating me for two years, both submitted letters explaining that I cannot maintain the sustained sitting position required for desk work. The IME doctor's report contradicts both of them and cites functional capacity testing I was never given.

I'm on day 6 of this and i honestly dont know what to do. Do I appeal internally first? Go straight to an ERISA attorney? File with the department of labor? My wife is working but this benefit was $4,800 a month and losing it is going to be catastrophic for us. Has anyone been through the two-year review and survived it?