Well, I replace my ATF once a year, just to be on the safe side. But i will never run the ATF on my tranny 40k or even 100k miles, as they are very expensive if neglected.

 

It is worth noting that it's not good to change over a high mileage engine to synthetic oil. I've heard stories of spun main bearings which is really bad, but I have not heard of any issues with a high mileage tranny changeover to synthetic ATF, only benefits.
That said, I would only recommend using conventional oil in the engine, possibly a synthetic blend, but don't go full synthetic on the engine oil at your mileage. It's not worth the risk.

IMO, I'd still give the synthetic ATF a shot. Synthetics are superior to conventional oil in every aspect, so why not give your trans the added protection. Although it may not solve your shuddering issue, at least you will have the added protection against long term wear. Last I checked, a new tranny for my 03 LTD was over $7k.
3 years ago, at 50k miles, I changed my trans over to Amsoil. I now have 128k, on the same fluid, and it shifts as smooth as the day I bought it.
At 140k, I plan to drop the pan and change the filter, but I'm definitely sticking with the Amsoil.
Also, some Montero owners report shifting issues in the cold. Not me, and it gets as low as 0F to -10F in the winter, here.