There is Brax, a differentiable physics simulator written in Jax. It includes Gym tasks such as Ant, Humanoid and more: https://github.com/google/brax
It is not full MuJoCo but a good base to add more features. Aside from position based dynamics (xpbd) it features motion in generalized coordinates using the same accurate robot dynamics algorithms as MuJoCo and TDS (Tiny Differentiable Simulator).