Now that I've had a bit of time to digest what happened immediately before and afterwards, is there any possibility that I installed the passenger axle incorrectly? I ask because I have seen posts online where "when returning the car to park, the gears grind"
Is there anything in your lexus minds that remind you of that situation? RD21 no movement in the car. Acts as if it's in neutral. When returning it to Park, the gears grind (also, when it's in RD21 there *is* a slight mph increase which I noticed when putting it back in park.

Could my axle somehow be the culprit for what I experienced today?

 

With further research I think I've answered my own question. On Saturday, when installing the new balljoint, I bet I didn't seat the passenger axle correctly. Another website had that specifically.. as quoted below..

"the car then sounded like it slipped out of gear…The engine would just rev up and not go anywhere. Same for reverse.
Also, when I tried putting it into the “park” gear, it made a loud, continuous grinding sound, until I turned the car off. If I keep it in Neutral and then start the engine, the noise doesn’t happen."

and the answer on that site revolved mainly around this statement

"Your transmission is turning but the power is not getting to the drive axles. The grinding when you put it into park is what shows the transmission is getting power.
Something in the differential part of the transaxle assembly is out of whack. Me thinks the backyard mechanics have missed something. It looks like a tow to a mechanic is in order." I think we can agree there this backyard mechanic done hosed it up.

"I am going to guess that the splines have slipped out of position and that this is causing the drive axles to get no power."

When I get home from school I will take that side apart and reseat it the best I can. Let's hope that's the issue. May be some banging involved (using wood on the end, of course)
-John