Will the wii autoreformat my sd card that i use in my dsi and erase everything?
or can i interchange it with no problem?
also i remember reading that Excite Truck lets you play your music in-game but that was before Nintendo switched to AAC format, does it still work with AACs?
No you can't format SD Cards through Wii.
As I read in the manual of the ExciteTruck game; it can read up to only 100 mp3 Soundtracks. They cannot be AAC.
Photo Channel, however, can read AAC Music, but again it can only read 100 and in ABC order.