Matt,
You sound in a worse state than me dude. I have a ruptured disc in my lower vert ( I think no.3). Have had it since 2005 and at its worst every step I took felt like someone sticking a knife in and twisting it.....don't mind admitting I was nearly greeting even for a big lad like me ! It won't get better and the only thing I was offered was a big Op to staple my spine to my hip. No thanks ! Fishing really can aggrevate it but this reflects me stalking fish, crouching down below skylines, walking on uneven banks etc. When I get home I am often in a fair bit of pain.
To help I try to limit my fishing time to only the peak feeding periods and this year have only been casting (dries) to rising and feeding fish and these 'economy' practises have certainly helped. Wading deep can also burger up bad backs so consider not wading if you can. I still go home 'as stiff a boad' and in quite a bit of pain, but I am usually ok after some of my stretches and physio exercises and can go out again the next day to do it all again. Like you, there ain't no way I am not going fishing so it is a question of pain 'management'.
The Simms 'Backsaver' definitely helps to a point on these punctuated sessions I do. You also get supports that specifically target the sacro-illiac area which may offer more support depending where the pain manifests.
Cheers,
Lindsay