Based on my hiking in scouts to include Philmont (10+ miles per day over 10 days with 35+ pound pack)-I think there are at least two components to your question
1. The shorts- My immediate thought went to Prana as their long pants i.e. Zion stretch are a go to for many folks. I would be highly inclined to go full synthetic so they dry fast and well.
2. The underwear- Ex Officio
Let us know what you do.