A load test tool to monitor? You probably mean a monitoring tool.
We basically use detector and some other system utilities to monitor DB2 and the DB2 subsystem during our tests. Most of the monitoring is done manually by our DBAs and System Support people and on occasion we can check the transaction log.
There are many other tools in the market by comapnies like BMC, IBM etc. that will give you some more monitoring possibilities. But in the Mainframe world everything is very expen$ive.