i have a 2001 5.4L 4x4 F150 7700 "automatic". i hope that this isnt the tranny, but when i accelerate (baby into it) at 40mph-between-55mph the truck starts shaking(almost feels like a big missfire or if the gears are slipping or missing but then feels like they are grabbing again right after "it slips"). but if i put the petal to the metal, i go threw gears just fine and got to 80mph just fine. and just to double check, i turned off my overdrive and then accelerated like normal and it was fine too. i just got my trucks' spark plugs and boots and coils replaced on MAY 4th 2008 because i thought i HAD a miss fire. i also looked at my differential, it has fluid in it,but it also has shavings and the fluid needs to be sucked out and changed with new. i need ya'lls help more than anything right now, if you have any qestion about my explanation ill be more that happy to give more detail if i can. thankyou and God bless.....

 

That is the range the torque converter locks up, sounds like there is a problem with that lock up.

 

are you sure of that.???