Ok, first things first no I don't believe the "zombies" are coming however like th point was made already some of the tactics would work well with natural disasters, pandemics, government meltdown, etc.
I would also like also point out no I am not an "expert" however a lot, and boy do I mean alot , of tax payer dollars were spent on training me how to shoot, drive, and survive with and in most ennviroment. I would prioritize in this order: transport, supply, defence, resupply/offense. Take it from the special forces, I will us Marine Force Recon as an example. Yes they have acess to every weapon system know to man, yet they travel very lightly armed, why? Simple answer they are not designed to fight a toe to toe fight with large opossing forces, if they make contact it is the "violence of action" they bring to bear and they break contact. So I would look at making my XJ, assuming you only have 2 maybe 3 people to transport with more I would look at larger suvs/trucks. Make it survivable, you don't need to worry about IEDs but simple armor, like it has been suggested expanded metal over your glass, maybe even replace all but the windsheild with Lexan? Then I would beef up you front bumper, winch would be nice but I would think in this regard it may not be a good idea. Under belly skids, you may have to crawl over things that you normally wouldn't think, conrete planters, park benches, trash cans etc, you don't want to get stuck or worst have a brake line get snagged. Low center of gravity is very important, mulit vehicle convoy is great and all but again like was pointed out "shit happens" so look at keepin weight low just as if you where to pack a bag for hiking keep heavy low and centered then work up and out. A trailer would be nice, I would go with a pinel hook style maybe base it off an old d35, Chrysler 8.25, etc with a 5 on 4.5 and spares loaded as previously suggested one on the truck two on the trailer. Maybe even on board air? You can patch a tire quick fill it and roll. Filtration is important not just water but your fuel source too, a hand or electris pump for both and maybe some of the storage tanks like you see farmers put fertilizer in on the side of tractors?
Now for the supply effort I would say pick three spots, all in different places but not entirely if you head to one and its already over run you don't want to go back where you came from, plan enought food, water, fuel to get you "there and back" by doing that you have everything you need to move if plans have to change, the identify places you can cache supplies at, storage facility? A cabin in the mountains? Make that your second goal but I will cover more in the "resupply" part. I believe we use use 4 gal of water per day as a planning norm in the military, and a gallon of water weighs in at 8 pounds a pop!
On the weapons side I see everyones points but let's not look at this in a vacuum, military/national guard will probably be mobilized this gives you potential acces to 9mm, 5.56 nato, 7.62 nato, and .50 bmg, with assortments of fragmentation grenades etc. I would pack a pistol chambered for 9mm, I know when compared to 40s&w or 45cap its crap but when its that or noting I have seen it put down bad guys. An AR style rifle that is chambered for 556nato & 223rem is the way to go, and yes they are NOT the same bullet chamber pressure is higher in the 556 and will destory any gun not rated for it. Maybe a bolt action hunting rifle chambered for .308 win(it is 7.62nato) but not really for defense more for staying resupplied. And my personal favorite a ruger 10/22, ammo is light and every Walmart, gun store, sporting good outlet from here to maine has 1000s of rounds and yes .22 will provide a killing head shot, you will need to double tap but it does the trick plus when you make it to the final safe zone it works great for small game hunting, and it can easily be suppressed(no such this as a silencer)
Resupply/offense: resupply is simple some small game, edible plants, etc. Why offense? Well let's say there is a small population of people near your hide site, if they are "zombies" it is easier to take out 20 zombies then keep pushing mind you this is something you need to think about if you underestimate your "enemy" you are going to be suprised.
That is just my ideas mind you the best thinking is to stay fluid don't expect 50% of your plans to workout, expect the unexpected.