I forgot what the "maximum percent difference" you can have between front and rear gears, but if you have something like 4.10 and 4.11, you're OK. I remember the old 70s Ford F250 had 4.10 in the rear and 4.09 in the front. You run into trouble when you start matching 3.73 with 4.10 or 4.10 with 4.56. Also, your front and rear tires have to match in height as well. Not just the height stamped on the sidewall - if you are running different tires front and rear, you need to get the manufacturers spec. sheet on the tires and make sure the actual heights match front to rear.
Good Luck,
Eric M.