I think I found a good treatment!
To clarify, I'm referring to antiperspirant stain, yellow and gross, particular on summer's white garments. But in this case it is not built-up and hard and crusty.
I had a box of Biz, that I had used a few times as a laundry additive, and was generally not all that enthralled with....
But I followed the directions to make a "paste" for stain pretreatment. I wet the stain, scrubbed it in with a toothbrush, wet again, let it sit for a day, then laundered in the Admirpool as usual.
I must say I'm quite impressed! I followed this method on a few other shirts with older stains, and they came out, but some took 2 treatments.
My faith is renewed, and my small investment in a box of Biz (on sale and with a coupon of course) seems to be worth it.