It sounds like there has been a lot of turmoil, so maybe some of it is that. Moving is difficult particularly if there are no activities and few friends (or all new friends). At some point, kids may be old enough to figure out that they are different from other kids in that they live with Grandma & Grandpa, not with parents. They can be resentful of anyone keeping them from their "real" parents. I don't know if the 8 year old is cognizant of those things, but it's worth considering.
How long does this child's tantrum go on? You say it goes on for a long time, but are you observing 10 minutes of fury, or am hour? What happens at the end of it? Is he calmer, or is he still seething? How are you getting him to his room? Do you and your husband yell too? What is his demeanor at other times, how does he handle frustration in school, how does he handle other authority figures? How long has he been in counseling and have you given it enough of a chance to work? Are you and your husband meeting with the counselor as well? Is this the right counselor?
You don't say why you moved to this town that has nothing going on. If it was for an essential job situation for you and your husband, that's one thing. If it was not a well thought out change and if it was not well handled with the kids, maybe you need to re-think it. It would seem that leaving your husband is a pretty desperate move. You don't really explain this very much so it's awfully hard to make suggestions. Is there other stress in the family? Are you considering leaving your husband for any other reason? Are the children seeing negative ways of dealing with anger and frustration that they are imitating?
I think you need some good help and advice right where you are. You want to cry, you are going crazy, you want to move. You could benefit from some support and some good parenting strategies that reestablish you and your husband as equal partners and deserving of respect, and ways to show these two children that they get much more of what they want when they are reasoned, non-violent and respectful. Somehow, they are getting the idea that this bad behavior is beneficial - they are getting some sort of a payoff, either because someone is giving in to them ultimately or because they are getting a whole lot of attention (even negative attention).
If there is something else really seriously going on with the 8 year old, you may need more help. A colleague's child had serious rages as well as learning issues and major food sensitivities. When they took steps to solve the food problems (and not by taking away more foods - they had already done that), the rages actually were the first things to go away. So a lot of really erratic behavior can be traced to other causes.
Good luck. Don't give up.