I have tmj. could it cause constant pain in neck and shoulders, mild headache (starting from base of scull), and plugged ear?

Yes,: Yes, it's possible it could cause all the things you mentioned. Please consult a local dentist with extensive experience and knowledge treating patients with TMD\TMJoint dysfunction.

It could: See your dentist or oral surgeon. Several treatment options available.

Yes: Yes. Could be other things also. Start with your MD. If negative, see a TMJ expert. Any dentist can be a TMJ expert with the proper training and experience. Most commonly, oral surgeons, prosthodontists, and orofacial pain specialists. Ask your MD, your dentist and your dental society for referrals.