A whining or rumbling noise that increases with speed usually points to worn rear wheel bearings, while oil stains on the rear axle casing and the inner side of the tires are most often a leaking pinion or diff seal on the Spicer rear axle. Both are classic wear points on the 140 and 164 after many miles.
When rebuilding the rear axle, the wheel bearings, seals and gaskets are the parts that most often need replacing. Note that the wheel bearings differ between early and later model years, so the year and chassis number determine which range fits.