If the estate is still being probated, you may want to ask for the court for assistance in settling impasses.
If not, before getting the lawyers involved, why don't you try mediation first?
If not then a probate attorney is recommended.
You need an attorney who practices in front of the New Jersey Surrogate's Court. Not all Probate lawyers do this, and as your case will likely need a judge, you might as well pick a lawyer who can not only review the probate aspects, but also go to court for you. This situation happens often, I see these cases all the time. Both executors have the right to retain their own lawyer, so you can retain a lawyer to represent yourself, as executor. The reasonable cost of this will be paid by the estate. There are several options, so you need to review the current situation with your lawyer and come up with the best plan given your set of facts. For example, I often will file an accounting with the court to bring the issues before the judge, which is often the cheapest and fastest way to handle the issues, but I would have to know more before I could tell you that in your case that is the best choice. Better to get started, as the longer these types of cases sit around, the more trouble seems to come up.