Your wheel bearings have failed!! I think these are an overlooked maintenance item on these trucks. The wheel bearings should be re-packed every 10-15K miles...more frequently if you wheel it where it gets mud and water as high as the hub. I have had 2 completely fail in the 12 yrs I have owned my Bronco. The first time was about 1 month after specifically having it serviced (bearings repacked); I was in grad school and had had my tools stolen from the truck about 4-5 months prior...so I couldnt do them myself. Anyway, they froze up while I was taking the back-roads from BHM to Mobile along hwy 43. I was about 15-20 miles N. of Thomasville in the middle of nowhere. Luckily, there were some nice folks who let me use their phone...very poor part of rural AL, I was relieved that they had a working phone. Anyway, since I was in the middle of nowhere, I drove as far as I could till it pulled REALLY hard (was already pulling)...turns out the bearing failed allowing the hub to lock and literally twisted the outer axle like a twizzler into two pieces this made the hub so hot it literally melted out and was dripping from the hub...I didnt realize that the axle was severed unitl after I was towed into thomasville, bought tools, bearings and hubs and prepared to do a quick on the fly "Bronco-fix"....I went to pull the tire off and the axle came out with it--seriously. I had to have another tow truck come all the way up from Mobile, so it could be taken to a known and reputable shop that could document that the bearing repacking was done incorrectly. The Goodyear ended up paying about $1500 for parts towing and labor. The second time, it was my own fault, I hadnt serviced the bearings in about 3-4 years (and alot of miles). I smelled a smell...the kind of smell you never forget...within two seconds I was pulling off the road. This time it was the drivers side. I replaced everything that time...u-joints, bearings, the spindle on the wheel that failed (kept as spare), brakes, etc. I had wheeled it a few times and was driving alot of highway miles as well at the time (50 mile commute), so its just maintenance we need to be aware of. Definitely remove and check the spindle...if you havent taken on this chore/maintenance yet, its a good time to learn. The Haynes manual has a good exploded diagram. Repack your bearings properly...I usually use a good bit of extra grease that I press into the hub around the bearings (along with packing the bearings well), to assist in keeping moisture away from the bearings. Some people will say its not necessary, its definitely messy, but I havent heard a great argument against it yet. Best of luck and sorry for the loooong story.
John