nebulous

wouldn't that partially paralyze you?

I can maybe put about 15-20% of my usual weight on my right leg, and it's blindingly painful to bend my legs / lean forward past vertical suggests it's not a normal strain. It's also been persistent now since Monday and comes in cycles (debilitating in the morning, passible by lunch).

Could potentially be appendicitis

Unless the pain isn't where I think it is

It's dorsally oriented, right of center on the small of my back, and relatively diffuse. It radiates in dull waves. Turning on the right side of my body is virtually impossible. Lifting either foot to knee height can only be done with great effort.

Do you feel a swelling when you slightly bend inwards and rub your lumbar?

If not, try grasping tightly where your kidney is and suddenly let go. If it feels painful to do so, you might have something to do with stones or an infection.

A herniated disc seems really unlikely unless you do hard physical labour all the time

What was the onset? Was it lifting or twisting? If so, potentially. Would you say it's more thoracic (upper back along ribs) or lumbar(lower back)?

Onset was immediate and very, very, acute after beginning snow shoveling (both lifting and slight twisting due to confined space and old pavement). Location is definitely lumbar - the epicenter seems about 3 in. above and 1-2 in. left of my right hip.

The onset makes me think skeletal, but the location makes me think a potential renal infection and/or acute appendicitis. I feel like the onset makes the likelyhood of either of the later to be unlikely, but medicine is not uniform, and I've heard of acute onsets of either.

I had a renal/urinal tract infection which began hurting rather suddenly, so it is possible your acute symptoms are caused by something alike.

But again, you mentioned shoveling so.. I mean, you should visit a doctor either way lmao. But I wish for you the best, since a simple infection is much less problematic than the former