Couldn't say for sure, never flown a 75.
But the b737s I used to fly had various avionics fits.
Some had Manual / Auto boxs: Manual = Pilot controlled vor+dme, Auto = assigned by the FMC for FMC radio nav.
Others had the same as on you picture, which worked by having frequency agile DMEs, so VOR and DME permanantly under pilot control, with DME frequency hopping to tune other channels to derive a radio nav fix.
DMEs in use accessable via the NAV STATUS pages if I recall correctly, so you can see if the FMC is either (1) trying to tune the aid, or (2) successfully interrogating the aid, or (3) actually using it for a fix. (a combination of colours and / or inverse video being used to denote which, according to the vintage of the CDU). You can also inhibit the system from using specified DMEs (e.g. if it is notamed unreliable, or clearly pulling your radio fix off), so there is a degree of control over the auto tuning.
Hope that helps,
CPB