A while back on an LDP I got it really bad. Inside top of the thighs - was that what you were alluding to? Sometimes known as Walker's Crotch. Bloody painful and uncomfortable. I got it because we set off on a hot sunny day in early September (as you can guess this was a few years ago as we didn't get many hot sunny days this year), and sweated so much that the tops of my thighs rubbed together. Every night I'd cover the raw patches with antiseptic cream but by midday the next day it was rubbed raw again. I tried using bandages but the sweat causes them to move.
Anyway, I finally found a pair (or rather 2 pairs) of trunks with longer legs which cover the tops of the thigh where the rubbing occurs, and made from high wicking fabric. I haven't suffered since, but can't of course guarantee that it was the underwear as I didn't always get the problem before.
I bought them from Field and Trek, but they're not showing on their website any more, however I'm sure one of the big names will sell them.
S.