YEs, the factory "posi" or limited slip (L/S) in the bronco is a Trac-Lok, which uses clutches or small dics to engage both axles. Simply put, an "open" diffferential is one without any traction-adding devices. When one tire slips, it gets all the power and the other tire sits still. A L/S or posi will detect the slippage thru centrifugal force, and engage the non-spinning axle to split power to both sides. A locker will do the same thing, but uses gears, air or other means to lock both sides together. A spool replaces the spider gears and both axles are fully locked at all times.
I've seen several of thhe broncos with bad traction in the rear with factory L/S due to the clutches being worn out. They can be replaced, and should be if you do a gear swap. For the Ford 8.8" rear axle, this is about as far as I would go to keep from damaging the gears when running taller tires. the weak point is the C-slips that hold the axleshafts into the center carrier, as well as the spot welds on the axle tubes.
Everyone has their own opinion and experience for the front. Stock set-up is open in most broncos, which is easiest on parts but remember the loss of traction and what happens/ One tire spins, the other sits still. If one gets in the air, it will usally spin. that leaves the rear tires to push you off the rock. With an open rear axle or a worn out L/S, you often have only one tire pushing the whole rig and you're stuck.
I like to run a full spool in the front D44 TTB, since it's the cheapest option when changing gears. Since you have to replace the carrier when going above 3.92 gears, it's only about $30-$40 more to go with a spool instead of a new carrier and perhaps a trak-lok or locker. It makes turning harder in tight trails, so you have to unlock one hub to "disengage" the spool for turning. but it's not that big of a deal for me, since I run with one hub unlocked until I need the extra traction of the spool and lock the other hub. It will be very ******* u-joints and axleshafts, and you will break one if you suddenly change traction.
For the 78 Bronco, I'm currently running it open, but plan to add a Detroit True-trac. This is a gear-driven limited slip that alot of local guys prefer for rock crawling next to the ARB air locker.
Lastly, there are some things you can do to upgrade or strengthen your stock axles. New gears, lockers, larger spline-count shafts, etc to help you out. But my thinking is why spend the bucks on the stock 1/2-ton axles when i can start off with 1-ton axles that are already stronger? Same thinking on the motor swap that I used. Why spend the bucks on another 351W boat anchor when I can start off with a 460?
Just my $0.02, and I would ask alot of questions and get several opinions from folks that have experience doing what you want to do with the Bronco. Have a vision of what you want the Bronco to become and work towards that goal. You can sell some of the used parts as you go to help offset the cost of upgrades, but you seldom get back what you've spent. I'm always amazed at how well a bone-stock bronco does on the trails, so go wheeling first, JSM84
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