Without trying to be offensive, this is something that you two should have worked out long before you got engaged. You knew before today that she was a PA and you knew before today what that entails. This discussion should have happened long before you proposed.
My own personal situation is very similar to yours, except our roles are reversed, I am the one with the "moving jobs", and she's the one who is forced to follow me around the country. I grew up on a large farm and ranch (10,000+ acre cattle operation and just under 10,000acres of crop ground), but I went to college, got a great job, and moved where the jobs took me. When I met my fiancee, within the first few months when things got serious, we sat down and had the discussion, "look, if I get a different job, we might have to move far away. If that works for you, then great, but if not, then this has been fun, but let's not waste any more time".
I miss living on the farm, and I miss walking out my back door, jumping on a colt and riding out across pastures with a rifle shooting every coyote I see. But I don't miss it more than I love my wife. Plus, the salary I'm making because of the job means I can provide a better life for my wife and kids than I could have if I stayed home on the farm. When the kids get school age, then I'll look for a different job where we can be closer to family and be out in a rural school. Also, the higher "city salary" has helped me buy more and more land "back home", which I rent out, that someday we can either retire on, or sell to buy land somewhere else to retire on.
For YOUR situation, My estimates (knowing a little about PA's and ag salesmen) is that 1) she's the majority bread-winner in the household 2) location will effect HER salary MUCH moreso than yours and 3) HER getting a good job early in life will open more doors for re-locating and higher salaries than keeping your ag sales job will. Not trying to be offensive, but those are the facts. Following her where her job takes her will mean a MUCH higher standard of living for your kids than staying on the farm.
In rural areas, jobs for PA's aren't widely available and usually mean lower pay for longer hours than the same job "in the city" would mean, and they'll be MUCH less impressive on the resume than the "big city" jobs. Despite your current opinion, there ARE rural areas around Chicago and there ARE Ag sales jobs around Chicago (coming from a guy working for a major ag seed producer "in the city").
A job isn't "forever". But when you asked her to marry you, you planned on HER being "forever". Moving to Chicago today and making a better salary for a while might give her leverage to demand better salaries later if you guys decide to move to a more rural area. Money might not be everything, but it DOES mean things like providing a better life for your spouse, or providing a better life for your children.
On the other hand, you might take this time to ask yourself why you wanted to marry her in the first place. Your post basically paints the picture that you either love hunting more than you love her and/or you are more afraid of leaving home than you love her. Either one of those alone kind of suggests that you made a mistake and were wrong about your own feelings. Or maybe you're just getting cold feet...
At the end of the day, my opinion is to change out of your skirt, put on your big boy pants, man up, and marry the girl you love and move with her to Chicago.