You are using an original Chevrolet truck axle? I cannot see thrust washers in either the parts or service manual. Either they are included with new gears or they are not used.
http://chevy.oldcarmanualproject.com...55ctsm0409.htm
This page shows the small gear (the pinion) part number as
370225.
Here is an Ebay auction for an
NOS 370225.
The same page above shows the side gear part number as
3652344.
Here are pictures of
NOS 3652344.
I am not seeing thrust washers in any of those images.
Is there any chance your truck has a different axle? Possibly a Dana from a GMC or a later passenger or truck axle?