Getting a pinion seal to properly seat (and seal) in the pinion shaft opening isn't always real easy to do. The issue is it's hard to get the seal 'started' in the opening since it is fairly rigid and it keeps wanting to cock at an angle. If the installer tries to pound it in with it cocked even slightly, the seal can be damaged.
Or the seal seating surface in the axle housing itself might have been gouged during the removal process.
What usually gets it sealed properly is to apply some good RTV to the outside edge of the seal like what the local Chrysler dealer sells which is less affected by the presence of gear lube like Permatex RTV is. RTV is like silicone for your bathtub & comes in a tube. I'd pick up a tube at your local Jeep dealer's parts department and hand it to the next seal installer to apply around the edge of the new seal. And if you don't have a Jeep or Chrysler dealership close by, get some Permatex Gear Oil RTV Sealant
which is specially/better formulated to withstand the gear lube that is inside our axles. About half of my local auto parts stores carry it, maybe more by now since it has only been out for a year or two by now. That particular stuff works better for this application than all the other RTVs do with the exception of the Mopar RTV I mentioned.
To me, the the key to getting a problem seal to seal is the use of RTV around its perimeter. And if the seating surface in the pinion shaft axle was gouged/scratched, the RTV (which stands for Room Temperature Vulcanization) should be enough to seal it.
They also need to apply a little bearing grease (not RTV) around the inside seal opening where it seats against the pinion shaft.
After the seal is installed, I have always had 100% good luck just tightening the pinion nut to 170-180 ft-lbs. as I was advised to do by Tom Wood probably 15 years ago. There is a long procedure in the factory manual for setting the pinion nut's tightness but for just replacing the pinion seal, I would just have him tighten it as I described. The mechanic should apply some blue locktite to the pinion nut so it won't come loose.
I'm certainly no expert auto mechanic but I've replaced a 6-8 pinion seals over the past 15 years & don't think any of the ever leaked afterward when I replace them like this.