Having owned a 1997 5.4 EB 4x4 with the 3.73 for 7 years now, I would say the biggest issue in areas that get winter and salt is rust. Problem areas on mine have been
*bottom edges of doors, especially the driver's door which gets most use.
*door sills
*AC condenser
*EGR pipe
*all 4 control arms
*the nose (seriously, had some unrelated engine trouble and the mechanic found out that the nose of the truck had not been connected to the frame for some time).
*oil filter and the oil filter relocation kit (the tow package equipped models have the oil filter relocated to behind the front bumper on the driver's side)
Most of the other issues have already been covered by others. Such as:
*The air ride is worthless, but a cheap Monroe conversion kit to coil-overs has held up surprisingly well on mine.
*Parking brake gets stuck from disuse. Or starts to drag. The only outwardly sign you may have of the latter is slightly decreased gas mileage.
*rear hatch and lift glass struts get tired and saggy
*rear vent window motors get stuck
*exhaust tick from warped headers (I had mine replaced with shorty headers. Yes, was expensive, but the NPI motor needs that slight extra torque).
*if the previous owner has not been keeping maintenance records then I would also get a full fluid change for everything including axles and the transfer case, a set of new platinum spark plugs, a set of new COPs, and 4 new O2 sensors.
*dragging rear calipers
*shifter OD button breaking off.
*shifter linkage rod breaking in half
* a persistent EGR issue is usually a DPFE sensor issue
*oil pressure dial and the warning light do not show any issue until most of your oil is gone, which is too bad, because the timing chains are no longer lubricated when only half is gone (learned that the hard way), so do check the level on a regular basis.
*if your front axle takes it's sweet time to engage in slippery conditions, but your ABS is fine, check the rear WSS on the differential, it may be faulty.
There are also some off-road related issues, such as:
*CV shaft boots come loose or break easily on mine.
*should you ever get your master cylinder brake lines changed from copper to steel, make sure they put in a spiral of maybe 5-6 turns to soak up vibration. My brakes started leaking from the master cylinder fitting three times before they figured it out.
Mine got about 13.5mpg on regular factory-sized AT tires. This dropped to 11.8 on 33" mud tires. Recently I have been driving like an old person, so it has gone up to 12.5. The upside of 33" mud tires is that it requires no lift, makes the steering feel perfectly weighted and on pavement it goes as if on rails (my record is about 1.5 miles of staying within lane at 40mph without me touching the wheel at all).