Do you have to swap front differential when changing rear axle from 3:42 to 3:73?

 

On a four wheldrive vehicle, yes the axle ratios have to be the same.

 

Like oilcanhenry said, Yes if you change the rear diff ratio you have to change the front differential ratio as well. If you don't you will blow up your transfer case when its put in to 4wd.

 

THIS.

Also, that's a fairly subtle change, most folks going to the trouble and expense of changing the axle ratios
generally jump a gear, like when I did mine. I went from 410 gears, which is a fine power combo, to the
342s, for increased gas economy.
Nothing wrong with it, it's just a lot of cost and trouble, to go one ratio up. Will you be doing much highway driving at all?