I've not heard the specific noise you mention but if certain it is from the leaf springs, there are a couple of things you can do for the short, and long term. 

Jack the rear of the truck up and place jack stands under the frame just forward of the leaf spring front mounts and remove rear tires for ease of access. If you have tall stands you can support at rear of the frame also. Using a tapered piece of hardwood, or plastic, drive this piece between the spring leafs spreading them and spray graphite wet to dry slip lube of some sort between the spring leafs. A heavy dry grease will work also but you don't want something that will draw dust or dirt. Do this with each spring leaf front and rear of the drive axle.

Second and more permanent option is using nylon slip pads adhesive attached to the leaf spring leaf ends These are very thin and tough but must be attached correctly to remain in place.

What I suspect you have is the spring leaves slightly rusting together when the truck sits. This rust bond breaks with slight road jarring and is very audible at times. I've seen this on many trucks and cars through the years although have never worked on your described vehicle type.

The second option is best performed by a spring shop as they will select the nylon pad thickness and have the expertise to install them correctly.

I've never found anything like that in a car taken apart long after it was built but over the years when still fixing wrecked cars from the 60's I used to find a lot of snack bags and such shoved into crooks and crannies plenty. Somewhere around here I have a "Frito's" bag from about 1969 with the "Frito Bandito" character on the package front. I kept it as found it in 1979 shoved into the core support, (radiator support these days) of a 68 Impala that was a friends father's car purchased new by him. Also found a "Giant Robot" character from the "Johnny Sokko and his Flying Robot" TV show of the day, (1960's) and the last thing I remember was a 1967 something TV Guide that was stiff but the pages could be read. This had advertisements from the Detroit, MI area in it so had been there since the car was built. I want to say that was a 1968 Chevrolet Biscayne car so probably and early production unit.

Here is something I found on the Johnny Sokko production:

https://www.vermontmoviestore.com/products/johnny-sokko" rel="nofollow - https://www.vermontmoviestore.com/products/johnny-sokko