2000 Jeep Cherokee Rear end bebuild?

I recently began to notice that the rear end was making excessive noise from the rear end while driving. This noise increases and decreases with acceleration. I decided to take it to a transmissions shop to have it looked at and was told that it sounded like the rear pineon bearings need to be replaced. My question is, is this a necessity or can I drive the vehicle for awhile before they absolutely need to be replaced? I am about to drive the vehicle from WA to MS in about a week and dont want to take the risk of having the vehicle break down on the road but I also do not have the money at the time to get it replaced. I was told that it would cost about $650 to have to fixed. Do you have any insight on what I should do?