A year ago I had my rear differential replaced. Do they change/drain the transmission fluid for this?
My dealer says my tranny fluid is pretty dirty and should be flushed. Just wondering if this is something I should do or if he's just trying to get some extra cash out of me.

I have a 99 Runner Limited, automatic with 204,000 miles. I don't remember getting the tranny fluid flushed in the past, but if I did, I did it around 100k.

I've also towed a boat with the vehicle but not in the last two years.

 

They shouldn't have to touch the transmission for a rear diff being replaced.
However, its is common for them to do a 30 point check or whatnot while your getting your vehicle serviced for other reasons.

If you haven't had the transmission fluid changed or flushed in 100K miles, yea.. your due for it to be changed/ flushed.
Some say flushing it with that much age on the trans can knock loose some gunk and really mess things up in there. Meaning, a flush could be bad but I'm not certain on that.
I think the recommended procedure in your situation would be to have the filter/ fluid changed a few times (3-4 times) over a few months.
This will be somewhat of a flush without putting the pressure in the system you'd get from doing a flush.

 

Being a tech, the usual flush they sell is a little on the low side of the amount of fluid I think you wanna run thru it with the typical T-Tech type machine. Ask them how many quarts and what type of fluid. At my shop we run 14 qts thru, not too bad.
If it's been awhile, I'd wanna run 18 qts (or more) of Max-life or better fluid thru it in my opinion. I would also agree with the post above in that you may wanna do this in small steps of drain and fills. Seen a few GM trans die quick after flushes, but that's GM for ya too, heh.

 

Considering you've gone over 100K without service and you've towed on it, I would do what scuba said and drop the pan and replace the filter. Then do a few more easy drain and fills over the next few months (just with the drain plug - not dropping the pan again). I flushed mine at 185K(I have ~202K on it now) and it was a good thing for it, but your tranny is a little neglected and you are one of those candidates in which a full flush may kill the tranny faster.

 

Thanks guys. I talk to the Toyota dealer about doing it this way instead of getting the full flush. Hopefully they will work with me.