Yeah so I've had similar issues with both my Rover and my Truck. Basically it goes berserk whenever I try to "TAB" into the vehicle. It bucks like a bronco until it ejects me out. I've tried restarting the game multiple times and it seems to happen at random as soon as I start getting into the game (around 2-3 hours). I've abandoned the suspect vehicles and built new ones and I have the same problem with the new ones.
3.) This time I was smelting some Aluminum before I tried to climb into my truck, but I can't be sure when the bug started.
4.) Like I said, this happens at a completely random moment that I can't seem to pin down to a certain occurrence. I've abandoned 2 save files because it's game-breaking at a certain point to not be able to use vehicles. I'll stay off until we can nail this one down.
Windows 10 Pro 64 bit
Geforce GTX980ti x2 SLI (6GB)
Intel i7-6700k 4.7GHz