Surge and No Power under WOT (Low Mileage Truck)

[f150_xl]

Ok fellas, I've searched for a while, found a few guys with similar problems and waited on replys there, but got no help...

So here goes:

Havin a problem with a friends 96. The truck is a 1996 f-250 4x4 reg cab 5 spd 4.10 ratio. So far I've changed the oil, air/fuel filters, checked the compression/turbo shaft play/compressor wheel/exhaust restrictions/ new GP&GPR (unrelated i know) and a few other tune up items. Everything on the truck feels tight and new and well kept. The truck only has 86k miles and the owner has taken excellent care of it. The truck is 100% bone stock besides a free flowing exhaust...

When free revving it from an idle to WOT, the truck will rev quite slow and sounds almost like its hitting fuel cutoff throughout the entire rpm range. During the sputtering, it will produce a very minimal amount of black smoke. Not blue/white/gray - just a small amount of black.

Driving it around with minimal throttle, it feels fine - doesnt shudder or miss, doesnt ever start hard, idles smooth...

Now heres the worst part - the truck will pick up speed great and feels fine as fast as you wanna go - until you push the pedal past 1/4 throttle. Then it literally feels like you hit a brick wall. The truck falls completely flat on its face.

I've been driving the truck for over a week and its not really a horrible issue unless you want to pass someone or merge onto the interstate - he says he doesnt ever notice it unless he's doing one of the above. I'm not trying to undersell it though, because I assure you this things a TURD compared to my IDENTICAL 1996 ECSB (also stock).

Any help at all will be appreciated fellas!
Thanks in advance.

As i said i've been around this truck for years and can assure you its been well taken care of as far as maintenance.

NOTE TO ALL

The truck has no CEL BUT i plugged my snap on up and ran a KOER & KOEO a little bit ago and got 2 codes -
one was a turbo boost pressure of some sort - I believe a P023?
and the other was an exhaust pressure sensor I believe it was a P047?
(I'll get the actual codes posted later this morning)

[OL' OutLaw]

pull the back pressure sensor and the tube and clean them.

[f150_xl]

Done. EBPV is functioning perfectly.