Agree with all of the above!
Check your vent to make sure it's not plugged, but the seals don't usually make a popping sound on their own. If it wasn't just a coincidental noise (which does happen) adding some mystery, then the noise was likely the bearing letting go and then the shift and movement causing the seal to leak.
Usually when a seal goes it's a slow ramp up. Starts leaking just a bit, then more over time. To get a big flow of gear lube all at once means either the bearing is toast, or your seal really gave up the ghost!
If you've read over any of the axle discussions though, you'll see that now's the time to do both seals and both bearings. Generally speaking, the rear wheel bearings are good for about 100k miles in our experiences. Some (like mine) can get old and still be working fine, but be out of spec in as little as 90k miles. While others can last upward of 150k. But when they get to that point, they're virtually on the verge of catastrophic failure, so should not be ignored.
More than one owner has lost a rear wheel at as low as 120k miles.
Now that there are at least 40, and up to 50+ years on an original set of rear wheel bearings, even the grease is old and tired even if the mileage is still low.
If I had a Bronco that only had 50k miles on it, but was still on it's original bearings and I wanted to drive it a lot, I'd replace them even if there didn't seem to be anything wrong with them.
So if you don't know your bearing's history, or if you know for a fact that they've never been changed, I'd say we're all glad you heard the pop and saw the leak before it got worse!
I guess the good thing about living where you do is you don't spend a lot of time at 70mph on the freeway.
Old tired grease and bearings don't appreciate the extra heat of a heavy load, towing a trailer, or high speed driving.
Paul
