my truck a 94 f 250 4x4 when I put it from park into drive it jumps then it makes a loud slamming clunking sound then after that it stops.it also shifts to fast and to slow when i drive it and when i get up to move fast on the highway its really slow shifting trough the gears to build up speed when you first get on from the ramp .I know it needs a rebuild but why is it doing this?