To keep you moving though the goo, your wheels and tires need to be in contact with the mud as much as possible and transmitting power to the ground whenever they are. That means mud trucks should have softer-riding, longer-travel suspensions than other 4x4s because this terrain can require momentum and speed to make it through some obstacles. Soft-riding leaf springs may be the best suspension design for mud because they will tolerate the harsh, wet, grimy environment better than fabricated four-links with coilover shocks and rod ends.
Body/InteriorMud bogging does not require body armor. It requires very little sheetmetal at all if you don't mind getting slimy. Ideally the body of a mud truck would only enclose the engine and driver and concentrate the weight of the truck on the middle-to-rear portion of the frame. You want to keep as much weight as you can off the front end to help the truck float over the mud. Some of the most successful mud trucks don't even run winches up front because they weigh too much. Instead they rely on others or use a rear-mounted winch when they get in trouble.
To build a mudproof interior, the carpet should be torn out and any wires/computers on the floor should be rerouted to higher ground. We like the idea of covering the floor with spray-in polyurethane (it's not just for truck beds!), but a rubber mat or a few layers of thick paint will work too. You're going to be hosing the interior out when you get home, so put a drain hole in the lowest part of the floor to make cleanup easier. If you care about your seats, cover them with a waterproof seat cover like those available from Wet Okole (888/24OKOLE, www.wetokole.com). Even if you don't get stuck yourself, you will be tracking mud into your 4x4 when you get out to help others with trucks that are less capable than your own.
Engine/Transmission/Transfer CaseThe trick here is to have enough power or gear reduction to keep the heavy mud from bogging the engine down to the point that it can't turn the tires anymore. Small-block V-8s won't handle the stress as well as a big-block of the same horsepower level, and they need lower axle gearing to generate the same results. Don't rule out modern diesel engines that are capable of tons of torque (our Power Stroke loves the mud!), but do keep in mind that the extra weight will push the front end into the mud. Fuel injection is nice if you already have it, but we don't think it's necessary for mud. Instead focus on keeping globs of crud from getting thrown onto the air cleaner, or even add a snorkel if you cross through deep water.
As always, transmission preference is up to the driver, but if you run though long bogs and need to shift, an automatic has advantages. Make sure to install an auxiliary cooler where it won't pack with mud, and keep an eye on the ATF temperature with a gauge. Regardless of what kind of transmission you do choose, don't waste your time with an overdrive unit because you won't need it. Take a few minutes to install the factory flywheel or converter dust shield to keep everything clean.
We know few of you will have enough power to spin mud tires in high range so don't go crazy with a 4:1 transfer case, as it won't give you enough wheel speed. And remember to run vent tube lines from all drivetrain components to the highest point in the engine compartment.