Kerosene would be fine, just don't use them under load. when they are full of kerosene. Putting it up on jack stands and letting it spin for a couple minutes should not harm anything and might help get it cleaner. Mineral spirits also would work. Drain well, preferably overnight. If you have the top cover off the tranny you can also use spray cleaners when you are finished with the pre cleaning.
Save the drainings once you flush it and let it settle, then reuse it to clean up what you can reach with a parts cleaning brush. If you have a parts washer just use whatever fluid is in that. If you don't have a parts washer you should have plenty of kerosene for cleaning with when you are done.
