RE: Rash
As far as the rash, stay away from ALL cotton products. Cotton has no place in the back country other than maybe blowing your nose. Look for an antimicrobial type material and also look for a boxer type brief where there is no elastic band in the groin area.
Make sure that if you use wipes, there is no alcohol in the wipes. I stay away from wipes and take a bar of soap and a washcloth with me. It can be colder than heck jumping into a creek, but even after a 2 week stint in the backcountry, I don't stink. To me hygiene is huge on backcountry hunts.
Chlorophyl tablets can be used starting 2 days before your hunt and into the hunt. These tablets are amazing and cut way back on the odor that you produce.
Your rash could be from the rubbing but may also be amplified by bacteria build up....Get yourself some new shorts and keep yourself clean!!!!