Can you get pubic lice from trying on jeans at a department store even if you have underwear on when your trying them?

Highly unlikely: It is possible to get pubic lice from shared clothing, but the likelihood that a person with lice tried on the clothing for a few minutes before you decreases the odds a lot.

Possibly: Pubic lice or crabs (Phthirus pubis) can be spread by vector transmission. Wearing someone else’s clothes that are contaminated or infected towels, bed sheets or towels can give you lice. They will die fairly quickly if away from a warm body.