edited to add details
I give up trying to make sense of this and finally reaching out to this community for help.

My 2015 Trail Premium will occasionally get a really nasty vibration at higher speeds (65-75), but only when it's raining, but not every time that it rains. It feels like I'm driving on the rumble strips. If I slow down, it goes away. It's not at a specific speed (I can have the cruise set to 65 just fine, and when I go up a hill it will start vibrating) but seems to happen anywhere 65-75 mph. I have not tried to have it diagnosed, because it only happens occasionally - I've had the truck 5 years and put 60,000 miles on it, and it's happened about 10 times total. When it does happen, it's pretty scary and almost feels comparable to a death wobble. It doesn't happen any more frequently than it did when I first bought it used with 50k miles.

For reference, I have IVD Stage 2 (including UCA) set to about 2.5" front and 2" rear.

I originally had 275/70/17 BFG KO2 (e rated) and have since upgraded to 285/70/17 (c rated) that have maybe 30k miles on them (but still have significant tread depth). I have the original Trail wheels, no spacers. The issue has happened on both sets of tires (I hoped maybe it was a tire issue and the new ones would fix it). I don't notice any hydroplaning - yesterdays weather was especially wet with puddles everywhere.

The amount of rain or water on the road doesn't seem to make a difference. In the past I've been able to stop for gas or a break, and some combination of shutting the truck off, drive-park-reverse-drive, etc. seemed to make the problem go away. Yesterday that did not work (including putting the truck in and out of 4hi).

The issue is only in 2hi. I did consider putting it in 4hi when it was happening yesterday but not at that speed. And it only happens at 65+.

My alignment should be fine - I have it checked every 10k miles (lifetime alignment) and have not found anything terribly out of spec. That said I am coming due for a check.

No traction lights or warnings of any kind when it happens.

Any thoughts on what might cause this, or what to look for?

oof - this could be tricky. I will ask the obvious questions because details weren't in the original post.

what wheel and tire combo are you running? perhaps the tires you have aren't good in the rain and are hydroplaning - which is made worse by the suspension modifications you have

are you maybe getting water inside the rims throwing them off balance. maybe when you are slowing down or stop the water then come out of the rim

alignment good? is your trac light coming on when this happens? are you in 2HI or 4HI when this happens? does it make a difference.

if I was placing a wager, I'd say you have BFG KO2s which are known to be poor in wet weather and are hydroplaning / losing traction - the hill issues in your OP seem to offer a clue

Interesting theories! Also great questions that I should have addressed up front - I'll respond here and also update the OP.

I originally had 275/70/17 BFG KO2 (e rated) and have since upgraded to 285/70/17 (c rated) that have maybe 30k miles on them (but still have significant tread depth). I have the original Trail wheels, no spacers. The issue has happened on both sets of tires (I hoped maybe it was a tire issue and the new ones would fix it). I don't notice any hydroplaning - yesterdays weather was especially wet with puddles everywhere.

The amount of rain or water on the road doesn't seem to make a difference. In the past I've been able to stop for gas or a break, and some combination of shutting the truck off, drive-park-reverse-drive, etc. seemed to make the problem go away. Yesterday that did not work.

The issue is only in 2hi. I did consider putting it in 4hi when it was happening yesterday but not at that speed. And it only happens at 65+.

My alignment should be fine - I have it checked every 10k miles (lifetime alignment) and have not found anything terribly out of spec. That said I am coming due for a check.

No traction lights or warnings of any kind when it happens.

Again, it's so infrequent that it's been really hard to diagnose!

do you have another set of wheels you can try? so even if there's just a slightest bit of water on the pavement, barely wet, it will do the wiggles?

I still think the wheels /tires could be causing it. I wonder what the camber/toe looks like on your vehicle. there were a few lexus threads on water pooling in rims too. roadforce balanced tires?

have you tried rotating the tires to see if it moves from front to back, left to right or vice versa. perhaps it's only one tire

shaking in the rain - ClubLexus - Lexus Forum Discussion
Very strange front end shake and wobble.... - ClubLexus - Lexus Forum Discussion

These are both excellent posts and seem to mimic the issue that I'm having. Thanks so much for sharing those - I've searched just about every time it's happened, but then I forget about it until the next time.

It's definitely not every time it rains - we get a lot of rainy days here in NC and it's only happened a handful of times. But it's DEFINITELY in the rain, and doesn't seem to matter how much rain.

I typically rotate the tires every 5k miles, but have never really paid attention to which tires would be where when it's happening. I might have to start keeping a log book. They're due for a rotation now, so I'll be curious if it goes away for a bit. The tires are not roadforce balanced unless the shop did it without my knowledge when I had them installed...

if you can - get them road force balanced. it will cost you more and toyota/lexus (I believe) use road force as a standard. those big AT tires can be tricky and yours are on the big end. I think perhaps they are just out of balance or alignment and lose traction on wet roads the odd time coupled with the fact the KO2s have been known to be troublesome in wet weather

edit: I'm giving a lot of suggestions but I get that doing all of these things is not exactly practical or easy to do right away. but if you're taking it in might as well try