Check the axle vent hose, normally on the drivers side of the axle tube with a hose that goes up to the frame and makes an inverted trap. If that hose is missing/disconnected, you may be getting rain in it. I would pull the cover off, spray it with a full can of brake cleaner, it will evaporate fast enough where you could put the cover back on the same day. Doesn't seem to have a hose. It does have a little vent device on the axle with what sort of looks like a hat on it. Maybe 1/2" diameter and no more than an inch tall.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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