@DavidClapp -- Can you describe what you are seeing on the affected side of the negatives, preferably using some kind of magnifying glass?
Is it the the emulsion side that is affected, or the shiny base side?
I assume it looks something like dark smudges - are they grey/black or some hue or tint (color)?
If you lightly drag your fingernail lightly across the affected side of the film can you feel the edges of the marks? If so, are they raised or lowered?
Can you try to wipe off the marks using a soft dry cloth? Does a cloth dampened in water or alcohol dissolve them?
Have you been able to contact the lab that processed the negatives to hear what they have to say?
Edited to add: In case it is some kind of factory defect, Ilford (Harman Technology) should be notified to see if they have any other complaints with this batch of Ilford XP2. Harman's technical support staff are usually very helpful. They will probably want some numbers from the film box.
Thank you