What should I wear to an out-of-town rehearsal dinner?

It would be better to wear dress slacks to the dinner rather than jeans. You could wear the same blouse for traveling so you won't have to pack an extra blouse. Wear the same shoes for traveling and for the dinner - comfortable, low heel (not flats). With some clever arranging of things in your bag I'm sure you can cram those jeans in so that you have them for less dressy times. The key to packing is to economize and to pack only what you absolutely know you must have. When I travel I pack and re-pack at least 3 times before I have everything ready to go. Have safe travel and a wonderful time.