Not really a mod, but an accessory. When I got my t4r, I found the door armrest too low. Found this on Amazon and have been using it for months - it makes all the difference in the world for me. Much more comfortable, especially on long drives.
It's attached by velcro, and yes, I stuck the velcro adhesive directly to my limited's leather. The leather in these trucks are so impregnated and plasticized, that I'm not too worried about damage if I ever remove it...
I used an old sock, packed with syrofoam sheets packing, and slide it forward in thearm rest when not in use.