1996 is a series-2 engine with 10HP increase and a 10.4:1 compression increase. You'll probably find different pistons in there.
Here's the thread from Club Lexus on year-to-year changes...
Re-check your throttle cable again
Hi I have occasional reverse slippage as well. :mad: I am looking into this using the shop manual. So basically I am including a few things you can try, but realistically you need A) a shop manual and B) a referral to a transmission shop from your mechanic...