Is is appropriate and/or necessary to change the transmission fluid on a Toyota Tundra 2004 that has 108,000 miles?

If your automatic transmission fluid has not previously been changed, it would be a good idea to change the fluid at this point. With 100,000 miles on the transmission, it would also be a good time to drop the transmission pan and change the internal filter, too. That's a messy job, and one that I have started paying people to do--good thing you only do it about every 75,000-100,000 miles.

Ordinarily, it's a pretty good idea to change the transmission fluid every 50-60,000 miles, especially if you do a lot of start and stop driving or especially if you use your vehicle to tow anything. Fresh transmission fluid will probably help your truck shift a bit more smoothly. More importantly, the fresh dose of lubricants and the like will reduce wear on the transmission bands. I'd rather change my fluid more often than have to rebuild my transmission. With proper maintenance, though, your transmission should last well past 150,000 miles; I've seen Toyota transmissions with well over 200,000 miles on them with fluid changes as their only maintenance. Some have made it close to 300,000 miles with only fluid and filter changes, but I wouldn't count on that for the average transmission.