When I drive off at a normal speed say from a stop light everything fine. When I drive off fast from a stop light I get 2 or 3 loud thunks from underneath the vehicle. Mechanic shops says transfer case needs replacing. Hi-low range change over works fine so not transfer case motor. Get the thunking sounds whether in high or low range. Front drive shaft recently replaced, rear drive shaft still original. Obviously this is torque related due to noise only coming when driving off fast! Any ideas