It is unfortunate, but parts breakage and four-wheeling go hand in hand. If you have not broken down on the trail yet, consider yourself lucky, because it is not a matter of if but when. Lesser-equipped vehicles have some glaring weaknesses directly from the factory, and those who have upgraded their vehicles with stout aftermarket parts often follow the siren song of difficult terrain that further taxes the vehicle's components. Regardless if you are one of those people who loves to find creative solutions to problems or loathes getting dirty on the trail, the following will help you get home quickly, easily, and safely.
We carry the following items in our vehicle at all times, regardless of whether we're on the pavement or the trail. Most of them are small and inexpensive and can be found at the local auto parts store. They sit in a sturdy rubber tote that doesn't rattle as much as a metal toolbox and which is secured in the cargo area of our SUV.
We add the following items in an additional tote when we travel to distant trails, difficult terrain, or extended trips in the backcountry. They take up more space but are invaluable where there is no help (much less a parts store) within miles. We broke these down into three basic categories: tools, fluids, and parts. Remember to properly secure items in the back of your vehicle, otherwise they can become projectiles during aggressive driving or in a rollover situation.
Tools
Box- and open-end wrenches
Hammers
Hex-head wrenches
Various-size ratchets
Assorted sockets
Vise-Grips
Prybar
Pliers
C-clamps
Screwdrivers
Spindle socket
Hi-Lift jack
Max Ax
Ratchet straps
Graded chain and hooks or graded bolts
Air compressor or CO2 tank
Multimeter
Tarp
Coveralls
Latex gloves
Fluids
Engine oil and filter
ATF
Gear oil and pump
Power-steering fluid
Brake fluid
Grease and grease gun
Starter fluid
WD-40
Silicone
Oil-absorbing mat
Small container and funnel
Parts
Service manual
Fuel filter
Butt connectors and wire
Hose clamps
U-joints
U-joint straps or U-bolts
Fire extinguisher
First-aid kit
Kleenex pack
Flares
Jack
Lug wrench
Heavy leather gloves
Recovery strap
D-rings
Hose repair kit
Universal fan belt
Radiator stop-leak
Valve stems and cores
Tire plug kit
Zip ties
Jumper cables
Fuses
Baling wire
Duct tape
J-B Weld
Headlamp
Tire-pressure gauge
Leatherman or similar
Energy bars
Water (1 gallon)
Space blanket
Sunscreen
Bug repellent
Every vehicle will have failure-prone items that are specific to that make and model. We have covered some of the major models, but if your particular vehicle is not listed here you may want to ask a fellow enthusiast or the parts department at your local dealer what they see break most often.
Jeep CJ
Front hub and studs or bolts
Model 20 axleshaft
Ignition module
Jeep YJ/XJ/TJ
Dana 35 axleshaft
Spare TPS
Slip-yoke plug
Track-bar bolt
Toyota Land Cruiser
Birfields
Coarse-spline pinion
(third member)
Toyota Pickup
Birfields
Axle drain plug
Steering arm (stock solid axle)
Idler arm (IFS)
Suzuki Samurai
Birfields
Transmission shifter-seat
Steering coupler
It is important to have everything in your truck bed or cargo area secured so that it doesn't move around off-road. At best, you'll end up with some dents in your bed and scratches on your gear; at worst, someone will be injured by a flying object. | 
J-B Weld can be used to patch a leaking fuel cell enough to get off the trail and home. Permatex also makes an excellent gas-tank repair kit. | 
This is NOT the correct way to carry a Hi-Lift jack! You are better off borrowing your buddy's jack than just letting your jack sit loose in the back of your Jeep or SUV where it could hurt someone. |
Here, parts from a Hi-Lift jack are scavenged to patch together a hydraulic-assist ram mount on the tie rod of this Jeep. Sometimes fixes require you to get creative and use the resources you have at your disposal if the exact parts are not available. | 
Just like with spotting, sometimes you can have too much help when troubleshooting a problem. It is best for the owner of the vehicle to take control and for everyone else to only provide help if requested. | 
You don't get nearly as dirty under your rig when you bring a tarp to lie on, and small parts are easier to find as well. We often bring a poncho that can be used for either rain protection or as a tarp on the ground. |
When a steering bolt broke on this Wagoneer, the owner was prepared for the problem and had it fixed quickly. Knowing your vehicle's weaknesses will make you more prepared on the trail, but fixing those faults before you hit the trail is an even better option. | 
Our tools ride in a canvas Craftsman bag that clatters less than a metal toolbox would. We carry metric tools to service our Toyota; however, standard tools would be more appropriate for an American vehicle. Not sure what to carry? Consider taking anything you use at home in the garage when working on your vehicle. | 
This is the proper way to mount a Hi-Lift jack. It is outside the vehicle and solidly mounted to the tire carrier at two points. The Daystar isolator is a nice addition to keep the handle from rattling. |