Hello again, I looked into the problem some more. I think youmay have a problem with the low/reverse servo piston or rod. These are selectable by length of the rod. If your system is at the end of the tolerance, it will allow slipping. Here is the chart for the rod length of the piston. This is located in the trans pan and can be replaced by removing the 4 bolt cover that holds it in place. If you have no codes, I would start by installing the next longer piston assembly. A longer piston, will give more holding power in reverse, but you must make sure that the band fully releases. Your trans guy should be able to check this out.
Could you please verify what transmission you have in the truck? The manual says that you have a 5R55, but your vin number says that you have a 4R70W. I need to know which you have? The longer rod, how many grooves does it have near where the piston is atached?
This is the picture of the low reverse servo for the trans you have.
This is the cover and bolts to remove
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Let me know if this one works?
Band/Clutch Application Chart No. 601
A = Applied
H = Hold
OR = Overrunning
NE = No Effect
Line Pressure Test