Before I give you an answer, make sure the Mucinex D is actually Mucinex D...sorry if that is confusing, but there is also a Mucinex DM, which is different in chemical compound from Mucinex D.

Mucinex D (guaifenesin and pseudoephedrine) can safely be used with plain old benadryl (diphenhydramine), however you should NOT take EITHER product for more than 7 days. If your symptoms persist for longer than 7 days, see your doctor.
You should not take any other products containing any of the active ingredients in benadryl or mucinex d during the time you are using them.
A nasal spray may also help with nasal congestion. As long as it is straight saline, it may be used in conjunction with benadryl and mucinex d.