I am not completely sold on the "parts swapping" as a shop can easily hoomk up a scanner and view live data from various sensors helping to narrow problem down even if no trouble codes set.
What you are initially describing is a failing VSS or vehicle speed senosr. On the 4x4 its mounted to the transfer case. It sends data info to the trans control module to controll shifteing as well as speedometer function. If failing, you could develop the symptoms you describe
No way for me to be 100% without looking at scanner while driving and the problem happens to see if drop in the VSS occurs. It could be the sensor or the trans module or even the PCM ( computer) but from what we have seen and repaired, the higher % goes to a failing VSS ( speed sensor)
One would most likely pay for diagnostic wher they hook up scanner, focus on the VSS signal, drive till problem happens and then confirm bad VSS
Guessing? that is expansive and not correct approach via your symptoms. A VSS is basically an easy fix, not too expensive but best to confirm its bad rather than jumping in and swapping parts
Its a critical sensor. A failing one can cause several different problems to include exactly what you describe.Hoever, one cant rule out another problem so from my end, i have to consider the more common per your symptoms
I agree with odometer but,, due to circuitry in the IPC ( cluster) i am thinking its possibel but cant confirm. if anything they would swap the VSS. If still no go, The IPC is a computer as well. it could have an problem that back feeds the trans control module.
The dealer can perform a reflash of the IPC to attempt to help correct problems like this as well