My truck started making a loud clunking noise from somewhere underneath. It sounded like the transfer case. It was only at low speeds or when I let off the accelerator. On my way home something gave out and made a horrible clunking ( not quite grinding noise). Pulled over and it wouldn't move. I tried revving it up, every gear and every position in four wheel drive. When I put it back in park it still made a noise like something was trying to engage ( very LOUD rattling or fast paced clunking). The truck has 117000 miles on it. There was no difference in power or torque before this happened. I towed with no problem with my wife's Ford Windstar. Any help would be greatly appreciated. Thank you