>>3422543yeah so if it's not the AF-D version it's possible it's just an incompatibility.
I'd clean the lenses and the D750's electrical contacts.
Check that the AF-fine tune on the D750 isn't set to something weird. (try turning it off if it's on)
Check for a firmware update for the D750.
And maybe double check that the focus limiter on the lens isn't sticking.