I need to find a spare rim for my 99 4wd Ranger. What specs should I be looking for. And are they steel or aluminum?

 

Each year Ranger had more than one wheel option, 1999 probably had 3 or 4

Have a look here: The Ford Ranger/Bronco II Wheel Fitment Guide To Measuring Wheels

To see which wheels you have

Spare tire, even on 4WD, just needs to have large enough center hole(for 4WD hub) and correct Ranger bolt pattern, it doesn't need to match exact size, in width or diameter.

While you should never run different size tires on front axle and rear axle with 4WD, one tire of a different size to get you home won't hurt anything.
Every time you go around a corner the opposite tires on each axle have to turn at different speeds, so putting on a larger or smaller spare on one corner is just the same as vehicle turning a corner