Due to the nature of plate type limited slip differentials, the internal clutch plates will wear out over time causing the pre-load to drop.
This drop off in pre-load will feel like the diff is not locking properly, and will result in slower lap times and loss of grip in cornering as the diff can no longer limit the slip.
If your Tran-X differential is dropping pre-load then you will need to replace the plates inside the diff with a plate kit.
Below you will find replacement plate kits for the entire range of Tran-X plate diffs.