I have a 1977 GMC Sierra, just bought it and dont know if i am in 4hi or 2hi, there is no indicators.?

yeah chris, but test it anyway!

77 was the era where the full time 4wd was a common option. Does this have locking hubs on the front? This is the knob that sticks through the hubcap on the front wheels? Or is there just a flat metal cap on the hubs that stick through? If its got the flat caps then its likely you have the full time transfer case. The full time case shift pattern is 4hi lock - 4hi - N - 4lo - 4lo lock. When you are in the lock modes its just like a typical 4x4 truck where the front and rear wheels are locked and driven together. This is only used offroad or at lower speeds and extremely slippery conditions like mud or deep snow. The 4hi mode is the full time 4x4 mode which would be what we call these days all wheel drive. There is a differential between the front and rear wheels to allow it to operate safely on dry pavement. Low ranges is the same deal just at a lower range. This is only used for certain off road applications where maximum power is needed at very low speeds.

A lot of times there was a sticker on the back of the sun visor that talked about the 4wd.

Search for NP203 you will dig up lots more information. There is / are kits that converted this system to a more typical 4x4 setup. I remember hearing that MPG gains were not noticeable only gain was to save a slight amount of wear and tear on some of the front end components. Personally I liked the system.

203 Transfer Case Shift Pattern

Np203 Shift Pattern

I think yours is old school. You actually have to get out of the truck & turn the knobs on the front hubs to actually lock the front axles in on both sides

GONNA HAVE TO TEST IT OUT MAN

FIND A DIRT ROAD