Can I just bleed the rear brakes?

don't leave the lines dissconnected for long, fluid will drip out by gravity, no air will be able to go down the body line that the brake line is on, just do one side at a time. And don't let your Master Clyinder go dry, or you will have to bleed all the brakes.

ah, at some point, it might be nice to remove your front wheels. get the key or get those off (do a search on how to remove them without the key, if you dont know). tire shops can often do it too.

since you have new calipers, i would search around the shop for a mallable plug that you can jamb into the line to keep the drip and air contamination to a minimum. even rubber vac line caps can work.

i agree, you will likely not see air get back up front. generally, rear brakes have a high spot on them (i.e., where lines run over the diff, etc) that will bottleneck the air (air has trouble making toward teh master cyl from there). this same reason makes it challenging to get the air back out sometimes. i dont know that foxes are set up like this, but other RWD cars i have are. a miti vac helps with bleeding. you can also make a pressure brake bleeder (bleeds from mc) from a insecticide sprayer (Jrichker outlined the instructions once).
use new washers on the line fittings if applicable.
good luck.

What calipers do you have in the back?

Sometimes rear calipers have to be bled off the car, you can do this by gravity bleeding, or by putting a block of wood, or using a caliper spreader to hold the piston in. Most likely you have air in the caliper.