I had an old 3 speed that i would describe as jumping out of gear. It would make quite a bang when it did it. Not sure about slipping out of gear. I drove that transmission till it grenaded itself, then replaced it with a t10 4 speed. If your truck has a torque tube, transmission choices are limited. If you ever put an opendrive axle, you will have many more choices for transmissions including, but not limited to the t5, t10, muncie m21 and m22, sm420, sm465...
