Automatic Transmission - OD in my 4l80e doesn't work correctly...

My '92 K2500 4x4 has a 4l80e trans. My OD is not functioning right or left (ha). OD engages upto about 65 mph but then it downshifts to 3rd.

I have changed alot things, close to rebuilding the dang thing. I changed the solenoids because when it shifted, it slammed in each gear.

Here is a small list of the things that I have changed:
converter
solenoid (three sets, last one from GM)
pressure reg
od solenoid
cleaned the valve body
ECM
TPS :bang:

I recently scanned the trans and I saw that the speed that the trans was showing was about 20 more than the speedo was registering.
Is this correct? I asked a couple of mechanic friends but they did not know alot about electronic trans.

After reading alot different forums and websites, I have found out that the computer may think that it is towing something and that maybe the reason for the downshift.

Is there a method of calibrating the little box that reads the speed sensor (VSS)? :confused:

If there are any other suggestions, I would like to hear them.

Thanks for any help in advance.

If the PCM does not see lockup it may disable fourth have you had a scanner on it to actually see whats its doing ?

PM`d you performabuilt.

I did scan it a few days ago but the only thing I remember is the mph difference. i will need to go back to my friend and get the specs from the file I saved on the scanner.

Is there any specs I need to watch for?