I have a 1996 Ford F150 S/C 4x4, 5.0L with an automatic transmission (approx. 128,500 miles). I have developed an intermittant vibration that seems to happen most often at 1000 to 1500 rpm (the speed can vary). Shakes the entire truck, almost like driving on highway rumble strips. Doesn't do it all the time. Had a transmission shop flush the system, change the filter and fluid. They told me it was about the cleanest tansmission pan they had seen in some time, but that "it shifted a little soft." I do get the check engine light on occasion, but it doesn't coincide with the vibrations. Any ideas?
First idea is to find out what code is being set. Since your check engine light is on at times there will be a code stored in memory. The problem that is setting the check engine light might be the cause of the vibrations, too.
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.