The axle ratings on the truck is what matters so your trailer can only put X lbs on the truck. As far as trailer goes it's a licensing thing. Over a certain weight you get into inspections, higher licenses etc. I drove a 1999 F350 SRW licensed at 14,500KG GVW and was completely legal. That truck is nowhere near as strong as the new 1tons. I frequently see SRW diesels licensed at 16500KG GVW for commercial carriers in all makes. Although a 1500 may have similar tow ratings off the lot, you wouldn't even try to run those weights on a 1500, while a 3500 has no trouble with it.