Clipping negative voltage with resistor divider as load
Where should I put a limiting resistor?


The usual place to put this is directly in series with the op-amp output. However, you need to choose a value that isn't too high else you won't get the positive swing you need. You could also use a precision half wave rectifier circuit - it will only produce positive going signals. Here's one that is non-inverting and has a gain of unity: -



Should you go for the precision rectifier approach take note that you should use a diode with fast reverse recovery in order to obtain least unwanted distortion. A BAS16 or 1N4148 is always a good choice.

If you add only the diode (as shown in your question), the TL081 will limit current at about 20 mA to 30 mA. Maybe this is enough - maybe if you do calcs on heat dissipation you'll find you don't need a resistor.