You can push the pedal down and see if the truck will move. If it does the shoe(s) in the rear need replacing. Most likely the drum(s) will have to be replaced also.
You can buy just one shoe, they come one to a box. I had to replace one along with a drum/roter on my 2001 GMC, to get it to pass state inspection.
You will need to remove the caliphers and then the drum to check what needs to be replaced. The bolts holdng the calipher on are installed with
threadlock. You might want to heat them just a little to help break the bolts loose if you do this job your self. I was told that any of the threadlock products will turn loose at 200 degrees.