Are you sure you're using the correct white and the correct blue* to form the pair that is in use? If not, then you're using a split pair, which causes imbalance. Even without the AC power proximity, 630' of an unbalanced pair will pick up all kinds of noise, even nearby radio broadcasts!
* Or any other base color.
Many manufacturers of OSP cable, even drop wire, do not include a tracer mark on the whites to identify their matching base color forming a pair. You may be using the white that goes with the white/brown pair (or any of the others). Yes, it will work, but it will be noisy. You said that you checked for continuity on all pairs, but that just means that <a> blue and <a> white make it through.
Lastly, some Bell companies in the Southwest bury non-shielded drop wire due to their arid climates and soils, but that's the exception, not the norm. I've seen direct-burial CAT5e cable both in shielded and non-shielded varieties, but the shield is more of a copper foil. Regardless, grounding is NOT your issue. most ground faults on a working pair will cause all kinds of trouble, not just hum.
