Linda,
I had an L4/L5 fusion last summer. That surgery improved the back pain I was having, but did nothing to help the severe sciatica I get along the S1 nerve, especially when I sat. I was walking a mile a day by 4 weeks post op and the recovery really wasn't that bad, but my leg pain was still horrible. Also, I got sent to PT and the exercises and stretches they had me doing caused me bad pain in the SI joint on the left and kicked up my sciatica pretty bad. The surgeon who did that surgery said he couldn't find any signs of nerve compression that would be causing the continued sciatica.
So I started researching the SI joint since it was causing me so much pain when doing PT. I found a doctor who is an expert in the SI joint and he said he felt confident the SI joint was my continued pain generator. This last January I had the left SI joint fusion. Well, the recovery has been terrible. I'd have ten lurabar fusions before I'd have another SI joint fusion. I ended up back in the hospital because of severe pain a week after the surgery. My surgeon has said my recovery is completely not the norm though.
For the femoral nerve pain, I started having it after my SI joint fusion. It has improved some, but is still very much there. My surgeon was an orthopedic.
I also have been having problems walking since the SI joint fusion - as you mention stepping down causes the pain. When I walk or stand more than just a few minutes, my nerve pain gets going along S1, L5, and the femoral nerve. I'm trying to increase my activity by walking and doing some of the exercises/stretches I learned in PT, but if I overdo it I don't sleep that night due to the pain.
I still can't sit whatsoever. I've had a bad case of hip bursitis on my left side also which I guess is common with people who have had back surgeries.
There are actually a lot of ligaments that connect the SI joint area to the lower spine - look up some of the diagrams. There is a major one that runs to the L4 disc area in fact. It's possible that if your femoral nerve pain started after your L5/S1 surgery that the muscles/ligaments that connect this area got cut or damaged thereby causing your femoral nerve pain issues.