The easiest way tot "fix" the posts is to replace them.

Go to a real lumber yard (if you've got one available) & try to buy
some treated 4x4's that are already dry. If dry / treated posts not
available, go for the driest you can find (if you or they have a
moisture meter available, even better)

Ideally you want to buy posts under 19% moisture content, closer to 10%
would be better. I have purrchased treated sills (Hem-Fir) that
actually dripped liquid when crushed in a vise.

Also when selecting the posts, look at the ends. AVOID posts cut from
the very center of the tree. Try to get posts with the end grain with
a little curvature as possible. Less curve means cut further from tree
center. This grain pattern will tend to dry with less cracking,
twisting & warping.

Another possibilty (but a lot of work) is to "fixture" new wet posts to
restrain them while they dry.

cheers
Bob