Welcome to the Wonderful World of V-6's PLEASE CHECK AND ENJOY THIS LINK, it has every single thing you'll ever want oe need to know about our trucks. It was created by Jolly Goodfellow.
http://6066gmcguy.com/
I have a 1/2 ton GMC V-6 with a factory rear differential DANA 44 and that originally had a stock 3.92 ring and pinion. I swapped it down to a 3.23 from Precision Gear, that helped a lot but I had no overdrive, so I stayed on the low roads.....
I recently swapped out my worn out 4 Speed hydramatic to a 700R4 - if you consider that you'll need the factory or aftermarket trans adaptor to mate up an automatic. Once I made the swap and the truck had OD, I can cruise at 70+ and only turn 1500-1900. I tested at 80 mph and was only turning 1750-1800. See screen shot
Finding parts will sometimes be a quest but they are out there, reach out to both Kanter and EGGE Auto.