How to make white background keeping the existing drop shadow

I am working on several photos for a catalog in which the photographer has taken photos of the shoes on a white background with just a nice slight drop shadow. I didn't want to create a clipping file because I wanted to keep the natural shadow. When in photoshop the background is not perfectly white so it shows when bringing into a white page in InDesign. I tried adjusting levels in Photoshop but by the time I get the backgrd to read 0 the shadow almost disappears. I tried just selecting the backgrd and leaving the shadow unchanged but any selection makes the edges of the shadow unnatural. Anyone have any suggestions?
Thanks Chris

select the white BG with magic wand etc including the shadow.

Make a levels Adjustment Layer. This will automatically have a layer mas based on the selection, sho when you lighten the background the subject will be unaffected.

Now take a soft brush at about 30% opacity and on the layer mask, "paint" the shadow back to the density required.

Brilliant Chris J your a genius!!
It worked beautifully. Thanks a million, I have quite a few to complete in the next two days and you have saved me much time.
Chris D