neetchracer, A few of us on here have been through this. Mine is a long story and started somewhat like your's except I didn't find out until after a blow out on the interstate. I put our rig on a scale after the blow out and found about the same as you, almost 15K lbs on the front axle with a light fuel load. After talking to one of the new Sport Truck dealers I discovered we're not alone and a lot of the new rigs are coming with 18K front axles and tires. We did a full front axle swap on our truck along with new tires and rims to get to an 18K spec. I don't want say this is required but at a minimum you may want to consider switching to a set of 9K (9" wide) wheels and 315/75 x 22.5 tires. You can run the 315's on the smaller (8") wheels but you usually have to de-rate the tires a little. The trash trucks pretty much all run the 18K axles and wheels so this is where we found our parts. I will only run the Michelin 315 tires and replace them every 5-years. You can get a pretty good Michelin discount if you join FMCA.
There are some other tire options a little lighter or heavier but I felt the 315's would have better availability since a lot of the big passenger buses also run the 315's. We switched from 11R's to the 315. The biggest clearance issue was with the steering travel and we did loose some steering travel.
As for the axle assembly, some say these lighter axles are fine and I've heard the 12K, 14K & 16K sometimes use the same parts and just have higher spring rates. It's difficult to get a clear answer on this though. We had a Volvo only front axle under our truck and I couldn't get a clear answer from anyone on it's capacity so we swapped it for a true 18K used Rockwell assembly and then put all new hubs and brakes on. We also added dual steering.
I'd be happy to talk with you if you want or feel free to ask any questions here if you'd like.
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