Recently I noticed that it appears some LEDs on my passenger side tail lamp are not working. Is there a way to fix the individual diodes, or do I have to replace the entire lamp?

I have an 04 LS.

 

Entire lamp. The individual LEDs are not user serviceable. Check eBay for new and possible used replacements.

 

You need to change that side as a whole, they cannot be fixed.

 

There are two component light assemblies on each side. Here is the outer passenger side:

http://www.ebay.com/itm/04-06-LS430-Right-Outer-Tail-Lamp-Lens-NEW-genuine-Lexus-OEM-81551-50140-/171368437115?fits=Model%3ALS430&hash=item27e65ad57b&vxp=mtr