After riding in 2WD for a few minutes on pavement (with no leak) I switched to 4WD on a hillside yard. After coming back to pavement I noticed the axle was leaking again. When I inspected, the bolt I had placed was gone, and the hole was twice as big!
With your advice in mind, I began disassembling the left side of the axle, when something solid drained out of the hole in the bottom of the spindle casing (on the left in the picture). The ball bearings all seem to be present, but this looks like shrapnel from the bottom spindle bevel drive gear. All the teeth on the gear look OK, but I can't see the whole gear. I took the steering arm off, which exposes the top of the spindle case and found a broken fragment of the top snap ring.
Something bad is going on here. How did you know it might be a loose piece in the axle?
Six bolts hold the spindle case to the center portion of the axle. They are very tight. Five have yielded to persuasion, but the sixth was tough to grab. Now, after my best effort, the corners are all rounded off. Vice grips don't budge it. I have read on a recent thread that heat and wax can loosen a tight bolt, but I am reluctant to heat very much because I recall that cast iron can warp with heat. I was going to try to fix things myself, or at least deliver just the spindle case to the JD shop, but now I may have to deliver the whole tractor. Oh well. Thanks for the good advice.
OkieG