Will not accelerate past 40 mph

it is not supposed to go to O/D at that slow of a speed .get it checked either at your local dealer or transmission shop

Not sure what you have determined the truck is doing

If the rpm flairs the trans is slipping & done

If it won't go over 40 mph the exhaust converter is damaged
& blocked

Not much you can do,need professional help

I think you have an engine wiring harness issue going on here.

Check for a clogged catalytic converter.

Most chances that the clutch is weak

to me it sounds like the trans computer the trans computer is built into the main computer for the engine i would try that the local junk yard should have one

is this a 4cyl or V6...how many miles??sounds like the transmission isn't shifting to lower gears....is there any warning lights lit on your dash??

Hi,
Does it rev ok? If your motor revs high but still doesn't go fast, you could be having a transmission issue, where it's not shifting into high gear... I'm assuming it's an automatic.
I hope this gets you on track... Good luck.

Well if you would elaborate some more on what kind of car you have,...that would be helpful as my ESP is very low today....

one unit that operates ratio changes and is connected to the engine is that vacuum operated actuator on the transmission
it is possible that that unit is failing , vacuum hose is leaking and that will cause ratio changes as the vacuum varies
run the fault codes to check for wiring problems or VSS or TCM problems as well