My 86.5 Nissan hardbody needs help!

[47_MasoN_47]

Vehicle involved is my project truck - 1986.5 Nissan Hardbody (D21) 4x4. VG30i, King cab, etc.

I'm not sure if it's a bearing or what, but I was going down the highway (~60-65mph) this weekend after a camping trip and I heard a popping noise then something like the sound of someone dropping a few pennies. Shortly after this an absolutely horrid noise started coming from the front driver's side wheel. It's insane loud, and caused the truck to start shaking. I was about out of gas so I slowed down a bit and pulled into a gas station where I didn't see any fluid leaking or anything. As I was slowing down the whole truck shuttered and made another loud pop/grind noise and the noise from the wheel stopped. After I left the gas station the noise didn't start back up until around 35mph, which it continued to do until I pulled in my driveway. The truck stopped normally and it didn't seem like there was any resistance on the wheel (I could let go of the steering wheel and the truck didn't pull to the left or anything). I looked and it appears that there's a ton of grease that has sprayed out from somewhere behind the wheel. It's fresh (doesn't have dirt and crap stuck in it) so I'm 99% sure that it came from whatever went wrong. I touched the rim and it was hot enough to cause my hand to turn red.

So... do you guys think it's something wrong with the 4x4 (it wasn't in 4x4 when this happened) automatic hub deal or perhaps a wheel bearing? I'm not sure how to even find out what's broke.