Rebalance for an Effective Defense Uniformed and Civilian Employees Act or the REDUCE Act This bill limits full-time positions in the Department of Defense (DOD), in each of FY2024-FY2028, subject to specified adjustments to be made by the DOD Secretary, to a number not greater than 85% of the number of such positions at DOD as of September 30, 2018. Not more than 1,000 of such positions may be career appointees within the Senior Executive Service. DOD is authorized to offer voluntary separation incentive and voluntary early retirement payments to achieve the reductions but must use involuntary measures, beginning on October 1, 2018, to achieve required reductions in personnel levels if voluntary measures are inadequate.