First off, the difference between 4.11 and 3.55 is only about 14%. This isn't a great reduction, and going to an open driveshaft/different differential will offer you a great many better ratios. You can get as far down as 3.07 with 6 lug wheels and a 12 bolt rear end or down into the 2s with 5 bolt hubs. However, there are some things which are often overlooked (but which will in the process become concerns you have to address). Please let me point out a few of them:
1. You will have to buy a different driveshaft. Depending on the transmission you choose, you may have to measure and have one custom made or find one in a junkyard which happens to be the right length...and this can be a real needle in a haystack with the way junkyards are scrapping metal today. If you do find one, the trans yoke you use most likely won't be balanced to it. Count on a new driveshaft (balanced with new U-joints and a matched yoke shaft) costing around $400 to $450 delivered to you.
2. Your emergency / parking brakes won't be the same any longer. You will need to fabricate a new or unique cable setup so they will work now.
3. Unless you're fortunate enough to find a rear end with 6 lug wheels, you will need to find new front hubs (with 5 lugs) plus new 5 lug brake drums or carry two spare tires...one 5 lug and one 6 lug.
4. Your new differential won't mount correctly to your stock leaf springs without some work and even then it isn't likely to be ideal. The center pin in your rear springs isn't in the right place because in your closed driveshaft system, your stock differential mounts completely differently to your springs. The spring mounts provide the rotation needed due to the missing rear U-joint in a closed driveshaft system. In addition to this, even if you find a differential with leaf spring mounts on it and even if those mounts match the inward bound spring angle in your pickup (and I haven't seen one yet), you will have to remove those spring mounts and re-weld them after your pinion angle is established or figure out the pinion angle and shim those existing spring mounts in much the same way the front end is shimmed. If your pinion angle is wrong, you will wear out U-joints rapidly, you'll wear out your trans tail shaft bearing and your pinion bearing and listen to it howl and growl while that is happening. Wearing out U-joints rapidly can be very dangerous. If you've ever seen a vehicle lose a driveshaft at highway speed, you know how ugly it is. The sound of a bad pinion angle is sort of like whumm, whumm, whumm in a car with carpet and insulation and in a truck it is like driving next to a very old cement mixer truck.
5. Your old brake hook-up (the flexible line) probably won't match the new rear end and you will have to do some brake line work there. It is very probable you'll have to relocate where the flex line connects and this means welding on a different junction mount tab.
6. If you are changing the rear end, you may as well change the transmission so as to get a newer and better designed one (with at least all forward gears synchronized) and if gas mileage is your goal, you may want to go with a T5 transmission (which will either require finding a Jeep front shaft or making up a new clutch/pressure plate/throwout bearing combination). In this case, finding an 11 inch flywheel will be by far your best option and you may have to search for one of these. If you don't use the 11 inch flywheel, your options are to use a very small (8 7/8 inch) pressure plate/clutch combo to find one with a matching spline or have a rebuilder make a custom one for you if he can find the right hub. Clutch and pressure plate rebuilders have told me they think the 8 7/8 inch stuff is to be avoided on anything with more oomph than a 4 cylinder. One man in Waco told me he won't even rebuild them any longer. If you go with a T5, you must find one with a mechanical speedo output (and change the gears to match your situation) or use an electronic speedo. Or you might find a 3 speed with overdrive. Find one with a synchronized first gear if you go looking.
7. Whatever you do, save your old shock absorber mount plates. You'll need them because your "new" differential's shock mount system likely won't match your old one. You will also need new U bolts and they'll need to be really good ones.
Good luck. Changes like this are not that bad if you can do most of the work yourself and they make things better but there will be what we used to call nickle & dime expenses. Today they're $100 & $200 expenses, however.
Jon
