Yes, this is what a differential does.. Based on what you state, your differential is probably open. But, I have never thought that doing the "test" that you did could definitively determine whether you have an open or limited slip differential.
To really determine if your differential is open or "limited slip", you need to take the differential cover off and look (compare to pictures in the service manual).
Vehicles with a "limited slip" differential (stock) have warning labels -- for IH pickups, I think they are on the driver's side door jamb -- other vehicles could have the sticker on the inside of the glove box door.
Of course, after 35+ years, the original "limited slip" differential could have been replaced with an open differential -- and, the original open differential could have been replaced with a "limited slip" differential (no warning stickers).
And, the warning stickers could have worn off / been removed.
