In definition, the difference is very slight. Grammatically, resentment is uncountable and grudge is countable. So: She still harbors some resentment toward her old boss. She still harbors some resentment toward her old co-workers. She still has a grudge against her old boss. She still has some grudges against her old co-workers.

Resentment is bitter indignation at having been treated unfairly. in other words, it has to do with "Feeling" (bad feelings, hard feelings, discontentment, irritation) Grudge: A grudge is a feeling of resentment harbored. so like the other guy said , there is a barley any difference.