Based on the specs I've seen, in 1977 the stock Tcase with an automatic would have been the full-time NP203, and if you have a 400 engine you may have a TH350, not a TH400 (usually reserved for big block engines). However, GM did not always follow its own specs.
If you have an NP203 it should have a shift pattern like the one in this picture.
http://www.chuckschevytruckpages.com/drivetrains.html
The NP203 has kind of a mixed reputation, because the full time 4WD/AWD cut into gas mileage. However, for snow plowing and towing its nice because you are always driving both axles through the differential in the Tcase, and you can just shift in to High-loc to get full 4WD. If yours is an NP203 converted to part-time, the tell-tale sign is usually the addition of switchable locking hubs up front (full-time hubs were standard).
The Camper Special package was usually rated at about 8400 GVW, and you got a large front swaybar, extra wiring for the camper, and a relay in the engine compartment for camper power. The 2nd battery tray was optional, but often added with the camper package. Its a very useful GVW package because you can carry about 1.25 tons in the bed without exceeding the GVW (mine lists max weight as 2640 lbs), and the rear end won't even sag with that weight. You also don't need catalytic converters, and emission equipment on the engine is minimal.
However, if you plan to use that GVW you need to make sure you buy 10 ply rated (load range E) truck tires that can take the weight, not standard light truck tires. Load range E tires are usually a litte more expensive for an equivalent size, but its worth it to keep the load rating. 245/75R16 or 235/85R16 are common sizes of E-rated tires that work well on stock GM 16" steel wheels. If you read their specs carefully to pick the right one, even Walmart carries E-rated truck tires.
Bruce
