your torque convertor clutch is slipping. very common on the e4od. now i was gonna tell you to service the trans and drain the convertor but you already did this. so now there is no way around the problem but torque convertor replacment. its about 4hrs labor and the part is probably around 600$. if you pay attention it probably happens most commonly on a slight incline. this is when the convertor lokcs up. theres an easy way to be sure if its the convertor or not. just drive the truck. when it starts to vibrate step on the brake pedal very lightly. if the vibration goes away then its definitely the convertor. the brake switch unlocks the conveertor clutch. and the e4od normally shifts slushy. theve always been this way.