Has she said why she doesn't want to move?
What are your ages?
How long have you been together?
My question would be, how are you expected to develop a relationship with anyone, including her, getting home from work at 8p every night? This would be an opportunity to give you more 'at home' time. If she has a gripe that you are not developing a relationship with her children, then that is something that of course you will want to address since you have plans to marry her, but it's not a valid reason not to move, since moving will actually give you more time.
The fact that it would put you closer to your son is a double edged sword, while she should be happy that it would put you closer to your son and allow you to develop a closer relationship with him, she could also have fears that it would put you closer to your son's mother as well.
Having her reasons is not enough and is an indication of poor communication. Both of you are going to have to work on your communication skills and should sit down and put everything on the table...the pros and cons of the move. A mature attitude would be accepting that whichever choice has more pros for the family would be the way to go, unless there are one or two pressing reasons not to go that route.
If there's anything more you think I need to know or if you want to talk more, I'm here.