Thank you for your post.
What font are you using to create the form? Does that font exist on the second machine? Or, is there some font substitution? What video driver is being used on both machines?
Instead of using the underscore key, have you tried using the line tool and drawing a line? This will keep a static length no matter what font is used.
If you want a dynamic length, then consider formatting the merge field to be underlined.
Are you using the same printer for both machines? If not, then the page setup, margins, etc. may be different, and this would affect your layout and printout.
When you go to Preview Mode, this is exactly what is being sent to the printer driver.
Any other information you can provide may help.