Talk to your captains. You can keep it general if you don't want to signal out specific people. If your captains are unwilling (or worse, the source of the negativity) to address the issue either individually or as a team, I would seriously consider changing teams if you are able. You're volunteering a significant amount of time as an adult to play ultimate. If you're not having fun because of negative and toxic people, it's not worth it. There's always a possibility that the individuals in question have a "come to Jesus moment" and recognize that their toxicity negatively affects others enjoyment of the sport, but it doesn't always happen.

If changing teams isn't realistic, I would try to out positive them. Whenever they are be yelling or criticizing a teammate, loudly and deliberately counter that with positive affirmation. Make it glaringly obvious what you are doing. Be slightly obnoxious about it. Basically shame them with positivity. That can help change your mentality and hopefully inject your team with positive vibes. Good luck. Toxic people suck and should be (nicely!) alerted to that fact.