I'm about to go to 20" wheels. I have a 05 reg cab 4x4 6" skyjacker. Questions below

What kind of back spacing so I have no rub?
What size tire should I go with?

I'm lookn at these wheel

Moto Metal 956 20x9 (chrome)
Rockstars 20x8.5 (black)

 

I can't answer your back spacing question, but for tire size you should be able to fit 35" Tires.

 

Backspacing should be as close to stock as possible, which is 5.5"

 

Does the skyjacker kit widen the front track width at all? If so keep that in mind for your backspacing.

 

5.5" of BS sounds about right.

 

You should be fine with both of those wheel choices because they both have more than 5.5" of backspacing.

 

I think a 9 inch wide wheel with 5 to 6 inches of backspacing looks best. However, some lifts have certain requirements.