Visceral Obesity: Given the patient's history of non-insulin-dependent diabetes mellitus (NIDDM) and the description of the abdomen feeling very solid and tender to touch (TTT) in all quadrants, visceral obesity is a highly plausible explanation. Visceral fat accumulation is common in patients with NIDDM and can lead to an enlarged, firm abdomen.

Fatty Liver: Often associated with NIDDM, fatty liver disease can cause hepatomegaly (enlargement of the liver), contributing to a large, solid abdomen. Although liver function tests might be normal, imaging studies could reveal liver enlargement.
Ascites: While the labs are normal, early stages of ascites (fluid accumulation in the peritoneal cavity) might not significantly alter laboratory values. Ascites can be associated with liver disease, heart failure, or other conditions and could cause abdominal distension.
Abdominal Wall Hernias: Large hernias, especially if they contain significant amounts of bowel or omentum, can cause a solid, tender abdomen. However, they are usually more localized than diffuse.

Abdominal Lymphoma: Although less common, lymphoma involving the abdomen can cause significant enlargement and solidity of the abdomen due to lymph node enlargement or masses. It's crucial to consider this diagnosis due to its potential for serious consequences if missed.
Ovarian Cancer in a Male (if applicable): Extremely rare, but if the patient has a history of estrogen therapy or other specific risk factors, abdominal distension due to ovarian cancer metastasis should be considered, albeit very unlikely.
Intra-Abdominal Malignancy: Other abdominal cancers (e.g., gastrointestinal stromal tumors, sarcomas) could present with a large, solid abdomen. Early detection is critical for treatment outcomes.

Amyloidosis: A condition characterized by the deposition of amyloid proteins in various tissues, including abdominal organs, which can lead to their enlargement and a solid abdomen.
Whipple's Disease: A rare, systemic bacterial infection that can cause malabsorption, weight loss, and abdominal distension, although it typically presents with diarrhea and malabsorption symptoms.
Sclerosing Encapsulating Peritonitis: A rare condition that can cause abdominal distension and a firm abdomen due to encapsulation of the intestines with a thick, fibrotic membrane.