1. Check your J.D. for an engine heater by looking for an electrical 3 prong plug which you plug into a 110 household current. Let it "warm" for a couple of hours. The engine should start without using the glow plug. BTW bring your glow plug to a NAPA parts store and see if they can match it and if so, it'll probably cost much less than $50.
2. Your description on the axles tells me there's nothing wrong with it.
3. Each axle (you have tandem) is rated for 3500 lbs which equals to 7000 lbs. The load of gravel did not hurt your axles.
I think you're making too much out of this. Just load your tractor more towards the front, either backing it in or bucket first. I'll bet a dollar to a donut that you'll find that your fish-tailing will be gone.
Better yet, here's a Tractor forum you can post your concerns which I'm sure will be answered quickly... Good luck
http://jplan.com/cgi-bin/tractor/sea....pl?topic=750+