Forever Free by Joe Haldeman
Although this is his third "Forever" novel, it is closer to a continuation of Forever War than Forever Peace was. We have William and Marygay. They are veterans of the forever war. But it's enough years later for them to have two teenage children. They're making a simple living in a less than ideal planet.
The premise is that during the centuries they were fighting in the war the human race changed itself into something very different. The new version of people is known as Man. It is a collection of genetically identical "ideal" bodies which all participate in a group mind. The forever war veterans were given a choice of becoming members of Man or not. Those, like William and Marygay, who chose not to become Man were sent off to this frigid planet where Man allows them to subsist. Man does this in part to keep a genetic stock of old-style humans just in case they ever need any of their genes.
A group of the veterans develop a secret scheme to hijack a starship. They then intend to fly it far and fast to use time dilation. In this way they figure they will only age a little, but the rest of the universe will age tens of thousands of years. They then should be able to return to find that Man no longer dominates, making it possible for them to follow their own course. Much of the story involves the preparations for and the initiation of this plan.
However, things don't go as they wanted. Something strange and mysterious happens to the ship, aborting their journey. They return to find strange and mysterious circumstances there as well. It is as if the laws of nature were misbehaving.
In the end, the story requires questionable juggling in making its closure. Especially in comparison to The Forever War, it is not such a satisfying book.