Just noticed the used 2014 Ram 2500 doesn't have a spare tire. Can I put a 17" even though my other tires are 20"?

 

It's overall diameter that matters. They need to be the same, especially if you have 4x4, or posi rear. If not, it will seriously confuse the ABS.

 

 

FWIW, the smallest wheel size offered in 2014 for the 2500's was an 18. However, I don't think you'd have to worry about brake caliper clearances with a 17" wheel, since the 3500's had a 17" wheel option and (I believe) used the same diameter rotors.
If you've got the 20" wheels on your truck now, you probably have a 285/60R20, and the best match in a 17" wheel would be a 295/70R17, but options will probably be fairly limited since it's not a real popular size.
You might have better luck sourcing a used 18" wheel and putting a 275/70R18 on it, since they're both much more common.

$.02