YEs between the crankshaft pulley and the engine is the harmonic balancer. They can be rebuilt or replaced. I am guessing from your very limited info on your bronco listed under your name that you have a 1990 but I have no idea what engine you are running. For that year you can get the balancer for any engine from rock auto. Or you can have your balancer rebuilt by this company.
That flange you're talking about is probably the pointer for the timing marks on the balancer. The balancer has lines engraved (?) into it. When you set the timing, or do a tune up, you will line up the pointer with the marks. That will tell you how much timing you are running. Its a little more complicated than that, but that's the jist of it.
There is also a seal the is between the balancer and the timing cover. The photo shows the pulley still attached to the balancer. You can see a seal (the blue thing) installed in the timing cover that keeps oil from leaking out the front. If this were on the motor, the crank snout would come through the hole in the timing cover, the balancer would be bolted to the crank, and the pulley would be bolted to the balancer.
BKO = Bronco. If the seal is not leaking, there is really no need to replace it. In the pic above, the blue part of the seal doesn't do anything. The part that actually seals is the dark brown part that goes right up against the crank shaft snout.
Does your motor shake while the car is stopped, while the car is moving, or both?
If the motor is shaking, start by checking the motor mounts and tranny mount first. If they look to be in good shape (no cracking or tears in the rubber), then remove the pulley and balancer (AKA dampner) and visually inspect those. Make sure the pulley is still round. I've never had any problems with a balancer, but IIRC, there is rubber in the ford balancers that attached the hub to the outer portion. That rubber can wear out, tear or crack over time.
I haven't had to get that far back into my motor yet, but on the 5.0 you basically have to remove all the accessories to get the timing cover off. You're doing as much work as replacing a cam shaft without actually replacing the cam shaft.
yea the balancer is junk. STOP RUNNING THE ENGINE until you get this part changed. by continuing to drive it with a trashed harmonic balancer you run the risk of destroying the rotating assembly of the engine.
Take a really close look at your timing cover gasket. Look at the right side (pass) and the bottom for coolant leaks. If it looks fine just replace the balancer asy. Be careful removing the balancer so you don't nick the oil seal. If it leaks or you damage it, you can just remove it and tap a new one in with a block of wood and a hammer. I did and it went fine. If the timing cover leaks at all then I'd replace it because it will just get worse and possibly quicker now that you're working around it. Mine was leaking pretty badly so I replaced the chain & gears, oil seal, & the H2O pump too! The only problem with doing the cover gasket is that you have 2 bolts on each side of the cover, at the bottom, that go through the oil pan. You also have to replace the front oil pan end seal so use some RTV in the corners and be careful, go slow and don't get pissed.....lol...if possible! Weigh it out whether it's a must do now, or if not, just wait till you need to do a pan gasket or oil pump! :thumbup -Kevin-