I have a that lens and the focus throw is indeed markedly stiffer than all my other nikon lens. My 50/1.4 AIS is a loose one which makes for quick focus action.
I always took the 105s stiffness quite naturally, believing it to be intentionally designed for easier precision when focussing (it does have a particularly long throw from close focus to infinity, as most telephotos should).
My philosophy is this:
If it makes no weird noises.
If the entire focus range feels smooth with no cranks, cracks, or bumps.
If the stiffness isn't bothering you, how fast you can focus, or the stability of the camera (you want to twist the focus ring, not the entire camera with it!).
Then leave it be and be happy!
Ps: I got lucky. My 105 was bought by my father and given to me as a gift a year or two back. My father must have purchased it and used it once, literally. I was still in original boxing, manual, and in absolute *MINT* condition. No longer the case though.........