If the usual SI activation stuff doesn't help at all my SI usually calms down after I attack my glutes and hips with a hockey ball/foam roller then stretch and band-distract the bejaysus out of them a lá mobilitywod.
I've never had my leg seize up though, sounds like there could be more going on there? Teran Synge (1 Merrion Square) is an SI ninja jesus - talk to him.
As for what’s going on with it, this is the best description I have really:
Pain in my lower left hip when I bend down/sit down/get up/squat/deadlift..goes from bearable to sharp pain.
Constant ache going from that point down my leg
Weird pins and needles type feeling when I try to straighten out my leg
Leg shakes when I try to straight it out fully and feels like I’m over stretching it (which I’m not)
Constantly aware of it being stiff when I’m walking.