Ankylosing Spondylitis (AS) with associated osteitis or bone lesions: Given the patient's history of uveitis and HLA B27 positivity, AS is a strong consideration. The presence of lytic bony lesions could be related to the inflammatory process seen in AS, which can lead to bone formation but also, less commonly, to lytic lesions due to chronic inflammation.

Sarcoidosis: This condition can cause uveitis, lytic bone lesions, and can be associated with an elevated lambda light chain. The lack of FDG avidity on PET scan and negative bone marrow biopsy do not rule out sarcoidosis, as it can have variable imaging and biopsy findings.
Multiple Myeloma (early or smoldering): Although the bone marrow biopsy is negative and there are no CRAB features (Calcium elevation, Renal failure, Anemia, Bone lesions), the presence of lytic lesions and a slight elevation in lambda light chains keeps myeloma in the differential, especially considering the possibility of a precursor condition like monoclonal gammopathy of undetermined significance (MGUS) or smoldering myeloma.
Psoriatic Arthritis: This condition can also be associated with HLA B27 positivity and can cause uveitis and bone lesions, although the bone lesions in psoriatic arthritis are more commonly associated with joint disease.

Lymphoma: Although less likely given the negative bone marrow biopsy and lack of FDG avidity, lymphoma can present with a wide range of symptoms and findings, including lytic bone lesions. It's crucial not to miss this diagnosis due to its significant implications for treatment and prognosis.
Infection (e.g., Osteomyelitis): Chronic infections can cause lytic bone lesions and may not always show increased FDG uptake, especially in the case of certain types of bacteria or fungi. A thorough infectious disease workup might be necessary to rule out this possibility.

Langerhans Cell Histiocytosis (LCH): This rare disorder can cause lytic bone lesions and may be associated with a variety of systemic symptoms. Although it's less likely, the presence of lytic lesions without a clear cause keeps LCH in the differential.
Gorlin Syndrome (Nevoid Basal Cell Carcinoma Syndrome): This genetic disorder can cause lytic bone lesions among other findings like basal cell carcinomas and odontogenic keratocysts. It's a rare condition but should be considered in the differential diagnosis of unexplained lytic bone lesions.