Download it here, for free. I found a copy of the 1984 GMC Light Duty Truck Service Manual. It will of course work for Chevrolet trucks, not just GMC, and for years 1981, 1982, 1983, 1984, 1985, 1986 and 1987. It will also mostly work for years 1973, 1974, 1975, 1976, 1977, 1978, 1979 and 1980, though a few things will be differest such as windshield wippers, front sheet metal, interior, etc.
1984_gmc_light_duty_trucks_service_manual.pdf 78.01 MB
I don't see a copyright statement in it, but if there is and you're a GM attorny then please email me (danmantyla at gmail dot com) and I'll remove it! Don't sue me! :)
You can also BUY one! Please use this link so I get a few dollars kickback from amazon:
You'll also need these special tools, listed below. I'm giving this manual away for free, but you could help me earn a buck or two by using these affiliate links.