On my 2001 I have 235/70/15

 

175L on the odometer.
No doubt your suspension has sagged some over the years.
Replace teh shocks and see if that fixes the problem. I bet it does.
If not you may need to crank your torsion bars some to get back to factory ride hieght. They are a spring and subject to wear just like everything else. After you crank the t-bars, get it aligned again.

 

Well I looked a little harder today at where I was rubbing and found out it was only the mud flaps. So I'm gonna be removing them this weekend.

Also I looked at the door and the sticker said 235-70.... Ooops


(edit) If I get new shocks do I have to get re-aligned?

 

Yea, GM used several sizes on the stock trucks.
215 to 235, and there may have been some 205's as well.

 

Hahaha, see? I did the same thing.

New shocks don't warrant a new alignment. They're unbolt and bolt-in, no steering geometry should be effected.