I've got about 1 mm of radial runout on both of my back axle shafts (it is in the axles, not the wheels or tires). i'm eyeballing that runout-- i haven't run a dial indicator on it. it is about 2-3 mm on the outside edge of the tires. The right wheel also 'sticks' a little when the van is jacked up and it is is in drive and there's some bearing noise from it when it turns. You can't hear bearing noise inside the van and you don't feel any wobble when driving.
do I need to replace this right away? do both axle shafts necessarily have to be replaced?

 

That sounds like the wheel hit something hard enough to overload the axle, and possibly the bearing. If it's enough to cause sticking and make noise, you may want to replace it, and the bearing.

 

You can delay this up to maybe a week if you don't drive it at all.

If the bearing lets go, you lose the axle, and maybe the housing and brakes.

That's worst case. I drove my dads wagon with a bad one, and when I pulled it apart the next day the bearing fell out in pieces. No issue, it went back together ok with a new bearing. But it was mighty warm after only 5 miles.... Lucky I made it home.

 

I know from experience, that the rear axle bearings make a lot of road noise as they're wearing out. I had one shatter, and gear oil poured into the hot brakes. Made quite a smokescreen, thankfully no fire. Also cut a fairly deep groove in the axle. Keep in mind that the axles are different lengths for each side!