[mugwump]

And I have to reluctantly mention, this has worked for me in the past for the itching but it's a bit intense. Try high-dose Benadryl first.

Fill your tub with hot, and I mean just shy of second-degree-burn hot, water. Japanese bath hot. If you can put your foot in the tub comfortably, it's not hot enough. Put towels/bath mats down around the tub.

Submerge yourself as quickly as possible and hold yourself under. The heat will cause the mast cells in the dermis layer (mast cells produce and release the histamine which causes the itch) to simultaneously and immediately dump all their histamine. You will experience what the medicos describe as "exquisite pruritis" for about a full minute. Hint: when medicos use the word "exquisite", it doesn't mean what you think it does.

If you have a dose of poison oak/ivy/sumac as bad as you describe, you will briefly go insane with the itching. You WILL thrash around. (You did remember to put down those bath mats?) But as quickly as it starts it fades, and then you'll have about 4-5 hours of relief before the mast cells build up the supplies of histamine.

The keys are hot water, submerge quickly (or the leg you put in will itch so badly that you'll never get the rest of your body into the water), and be willing to experience 5 hours worth of itching in one minute.

Good luck.