I had exactly the same issue with one I bought from JXE Peterbilt 3 years ago. We resolved it this way: per discussion with Dana:
10 & 12K axles are essentially the same & use all same parts. Except: the tie rod ends. Change out the tie rod ends, put a new sticker on the door from Dana confirming a 12K rating and voila, you are in business. You do not need anything else- like expensive spindles. That does assume you have 12,000 lb. spring rating. Otherwise, you may have to change that as well. Talk to JXE in Waukesha if you still can't get the right answer.
As you may know by now, not all Pete dealers are created equal. London & Woodstock, ON. and Dermody, Grand Rapids are the best I've found.
Despite the OEM sticker saying a 31K GVW vs. 33K, I have never had an issue at any scale house.