If you're not going to be doing much offroad driving in your Jeep, then you can use the synthetic stuff, because it ;asts a fairly long time.

However, if you're going to be doing a fair amount of off-road driving that includes water crossings, then you will be changing the fluid in at least the diffs after fairly deep water crossings - maybe even in the other driveline components. That would be a waste of money then, so I'd use the normal stuff in that case.

Keep in mind that the vehicle was probably designed for non-synthetic stuff, so you're not likely to go wrong in using it. The synthetic stuff is nice in some situations, but it's certainly not necessary. Keep in mind that the profit margin is higher for the Jiffy shop on the hi-end oils, so of course he's going to recommend those to you.