Replacement axle?

On the other hand, I have my doubts about the value of going to a larger capacity axle.  Not that it wouldn't be stronger, but rather that that if you loaded the trailer consistent with the capacity of the axle you'd be overloading the basic frame and would just be moving the risk of failure to another weak point.  The frame and the axle, I suppose though I'm far from an engineer, function as a single unit and like a weak link in a chain all you are really doing by going to a heavier axle is moving the weak link to a different location.  

The Pod is a lightweight trailer that has its limitations and appeals to a specific market nitch.  It shouldn't be overloaded beyond its design capacity.  Attempting to increase its design capacity, in my opinion, is asking for problems.  If you are unsatisfied with the GVW of the Pod, a better solution would to be invest in a travel trailer that will handle the loads you wish to carry.  

I ran the load calcs on the frame. To keep things simple I  just assumed a uniform load from tongue to rear bumper. This tends to overestimate the weight on the tongue but I don't think that changes the end result too much, because the highest stress load on the frame occurs at the axle. 

Using my 0.1 inch frame thickness figure, I got a  bending stress at the axle of around 22 ksi assuming a trailer loaded to 3900 lbs. That only provides for a 1.65 bump load safety factor. Not real good. The other peak stress point is up front about 78 inches back from the tongue, but that point has an adequate safety factor of around 3. 

In reality, its not going to be that bad because the load on the frame from the axle is spread out somewhat and the trailer box will add stiffness. But either way, its a good idea to avoid putting heavy junk in the back of your trailer and an even better one to drive slow over bumps.