Windows 2000 uses the File Replicator Service (FRS), which is not compatible with LMRepl.
The Windows 2000 Resource Kit contains Lbridge.cmd to solve this problem.
The Lbridge.cmd script will push files from the Windows 2000 domain controller's Sysvol to the Windows NT 4.0 BDC. The script uses Xcopy, but I recommend you replace that with RoboCopy. Microsoft recommends that you schedule the script to run every 2 hours, on the Windows 2000 domain controller.
Before you upgrade a WinNT 4.0 PDC to Windows 2000, configure a WinNT 4.0 BDC to be the domains LMRepl export server. Remove this functionality from the PDC you are about to upgrade. Once upgraded, The Windows 2000 PDC will use LBridge.cmd to replicate the Sysvol's Export folder to the WinNT 4.0 BDC that is the LMRepl server. This BDC will replicate the data to the other downlevel domain controllers.