A 1.8 cm slip of l5-s1. cracking sounds in neck when i turn my head. arm pain in both arms. blood work was normal. are neck and back related? arthritis?

Degenerative arthrit: It sound as though you may have degenerative arthritis though other forms of arthritis may not have blood markers so a trip to a rheumatologist is in order. You did not mention your age or occupation as these may be factors.

Could be if....: Your are or have been a smoker. DJD of spine ( lumbar and cervical) are much more likely in smokers. But some are more prone than others! The sliipage of L5 over S1 is called spondylolisthesis. It is rarely symptomatic unless the slippage is 50% of more of the width of the vertebra.

Unlikely to be: related, other than there may be arthritic changes in both areas.The "slip" is called spondylolisthesis, and may be due to a congenital or acquired defect(spondylolysis) in part of the posterior part(lamina) of the vertebra.1.8cm is a fairly substantial spondylolisthesis, particulary if mobile ,degenerative arthritic changes can accelerate and become severe, with associated symptoms. See your doc