Although the original pulleys seem to have a slight visual difference between the two of them, both idler pulleys have the same replacement part number, according to our OEM replacement parts source. We were able to get OEM replacement pulleys inexpensively and quickly.
Now to begin the work replacing the pulleys. First, remove the belt by loosening the tensioner, located below the alternator, after disconnecting the battery or batteries (this particular truck has two batteries). To loosen the tensioner, place a 1/2 inch ratchet in the designated area towards the bottom of the tensioner, pictured here. Slide the belt off of the alternator, but don't take it all the way off unless it is being replaced completely.
We aren't body builders, so we used a large 1/2 inch extended ratchet for extra leverage.