I’m looking for something that’s extremely deadly to poison ivy but mostly benign on other plants (especially grass).
Any suggestions?
The requirement to be benign to other plants is going to be nearly impossible.
Your best bet is going to be to pull out as much as you can by the roots, and use Brush-B-Gon very specifically on anything left or whatever grows back. But it will kill other stuff that you get it on, including grass.
Vinegar – extra strength vinegar if you can get it – applied directly to the leaves of the poison ivy (try not to get it on other plants), if possible on a warm day while it’s in full sun.
Repeat if the poison ivy comes back.
I don’t know that there’s anything that will only kill poison ivy and not kill any other plants.
Nuke it from orbit – it is the only way to be sure (but yes, that may cause nearby plants issues 
)
Brian
Trimec weed killer should work and not kill the grass.
https://gvt.net/sites/default/files/files/Trimec-Crabgrass-Plus-Lawn-Weed-Killer-Concentrate.pdf
Another one is Momentum, but not sure you can buy that in small amounts. It can be expensive.