dForce can be such a hassle.
I suspect it's the "Fit to" redirect/autofollow that is causing the underwear to distort. Normally, the underwear will try to redirect it's shape to fit to the new character pose. That'll create odd looking results if you start from a dForce shape around the ankles.
You could try changing the "Fit to" to "None" and then manually position/rotate the underwear to each new pose.
Another alternative is to inspect which hidden underwear Parameters get changed and try to manually dial those back for each pose until it looks normal. (Click the little box with four horizontal lines and then select Preferences->Show Hidden Properties.)
In my experience, it's usually easiest to run dForce again for each new pose. Remember to set SubD and Smoothing/Collision Iterations to a low number (e.g. 1) before simulating to speed up the process.
There could be an easier fix but I don't use dForce enough to know it.