This afternoon, I thought I hit a tree stump. I drive an 06 GMC Sierra. A week ago, I discovered both front wheel bearings were shot. The dealer swapped the hubs under warranty since the truck's only got 8,000 miles on it. Today, me and three of my guys were trimming some trees. I got in my truck to leave and pulled forward about 10 feet when it felt like I slammed into a tree stump. The guy up in the bucket truck heard the "crash" and said it looked like the back end of the truck came up off the ground it stopped so suddenly. The guy on the ground yelled "What the h#$% happened?" I got out expecting to see my bumper wrapped around a tree stump but there was no stump. I didn't hit a thing. From the scuff marks on the ground, I'm sure that my right front tire locked up. I don't know if any others did too. I gotta tell you, it didn't feel like a wheel locked up. It felt like I slammed into something solid. Boxes in my bed went flying and I was jerked back by the seat belt. I backed the truck up (to back away from the stump that wasn't there) and it's driving fine now. Wheels turn freely, no unusual noises. Bearings feel good, brakes look fine, I'm wondering if the transfer case locked up for some reason. I'd hate for this to happen again on the highway so I'm gonna call the dealer and have them tow it in for an inspection but I'd like to know if anybody else has seen this before.