What is a reasonable charge for a tranny flush / band adjustment at the dealer? I'm coming up on 90k miles and don't know the history of my truck so I figure it is a good time to do it.

Rob

 

I don't remember exactly but I think it's somewhere between $100 and $150 (I'm thinking $140) to replace the filter and fluid and adjust the bands. I think it's a little less than $100 for just a flush (which Jiffy Lube or anybody can do).
Unless the fluid is burt, contaminated, or has been neglected, I'd just get the bands adjusted and filter replaced. A flush will remove almost all of the old fluid where as dropping the pan, adjusting the bands and replacing the filter changes most of the fluid but doesn't drain the fluid from the torque converter and tranny cooler and lines (if you have one).