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There have been many premature transmission failures mostly occuring during the manufacurers long warrantee, which Lexus stood behind. Your problems sounds like a common experience for vehicles with low mileage that didn't get taken care of during the long warantee period. It seems once you're past 7/70, you're on your own. If you have an established relationship with a Lexus dealer, they may work with you on the expense, especially if the dealer was doing the maintanence and wants to earn your future loyalty.
To fix it properly requires transmission replacement (with a more modern design) either new or rebuilt. That can be really expensive and the cost seams to float pretty high based on the vehicle's percieved value. Best to shop around. If you can't afford that, you could try having the transmission serviced by dropping the pan, cleaning out the broken and worn stuff on the pan and replacing the filter, and new TYPE IV fluid to replace your old yucky fluid likely overheated and burned, and see if that extends the life of your worn out transmission. When one part in the transmission gets overheated and/or worn, it mucks up everything else, and things go "downhill" really fast from there. See the messages about the fluid type used. If you go to an independent shop, make sure you or the shop has at least enough fluid plus an extra quart for you (it may require more fluid than the service manual calls for).
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