Re: Wanting posi...Which is easiest?
Cheapest decent alternative is to do as you are planning. Get a used 8.8 28 spline limited slip diff off eBay, and a rebuild kit. Rebuild it yourself, it's really easy since there are so few parts involved. Installing you self does take some time playing with painting the gear and judging the pattern (a piece of paper should come with the carrier bearings you buy that will tell you how to do this). However, you won't have any spacers on hand, nor in the kit. Which is why you will probably end up taking the rebuilt diff to a shop and telling them to install it. It's not a huge job, so they should do it for about 100 or so ish. If you decide to change the gears (very recommended for performance increase) then you will definitely want to take it to a shop and then it will cost upwards of $200 labor.
For any kind of street use vehicle I would NOT recommend the PowerTrax. It's a locker style setup, so what it will do it perform as an open diff when it starts slipping and then work like a welded diff when it's not. Great for off roading and strait line racing, not for street use.
If you want to spend the money on a good diff for daily use get a TruTrac, or a Torsen. Both of these style diffs are limited slip, but instead of being clutch based unit, it is a gear based unit, or a hybrid of both. The TruTrac is purely a torsion differential. It uses helical and worm gears instead of clutches so that when 1 wheel begins to spin it instantly reduces the torque on that wheel and transfers it to the other.
As an example of the real life difference from a conventional limited slip, suppose one wheel goes off in the gravel as you are accelerating around a road block while running from the entire police force.
In the case of a torsion differential it balances power to each wheel to maintain the same RPM at each wheel. So as the right wheel goes off into the gravel, the differential will sense the right wheel beginning to spin which will result in a reduction of power to that wheel, this power is then transfered added to the left wheel. So let's say that you put down 300 ft*lbs of torque. With both wheels on pavement without they can handle 500 ft*lbs before spinning. So one wheel can only handle 250ft*lbs be it self. So you put down 150tf*lbs to each wheel. But a wheel in gravel can only handle 75ft*lbs. As you put one wheel in the pavement, the diff transfers the excess 75ft*lbs to the other wheel. Now you are putting down 225ft*lbs on the left wheel, and 75ft*lbs on the right wheel. That means you are still at full acceleration with no wheel spin. But because the torsion differential does not try to hold the wheels together, but instead only balances the power, this means that in a turn the inside wheel is free to rotate slower than the outside as it is required to do.
In the case of a full locker (or spool, IE welded diff), both wheels are forced to spin together no matter what. So with one wheel in gravel you get the same result as the torsion differential. However, in a turn the wheels are not able to spin independently. So the result is pone of two things, either the wheels spin as a result of the tires losing grip because of a twist in the axle, or the axle breaks. This twist in the axle results in force at each wheel in opposite directions, it keeps building until one wheel slips and relieves the tress.
In the case of the clutch based (conventional) there is a % difference in which the wheels will still spin at the same rate. In most cases this percentage on street or mild track use differentials is less than 40%. This means that if the traction of the right wheel is less than 40% different from the traction of the left wheel, they will still keep spinning the same RPMs. However, in the case of one wheel on gravel and the other on pavement, the difference is significantly greater than 40%. In this case the clutches reach their limit and one wheel begins to spin, this spinning result in relatively all your power going to forcing a wheel to spin, and not accelerating. However in the case of a turn, this % difference allows the wheels to spin independently with only some interference. This interference is equal to that % difference. So you now see that as you increase the strength of the diff (ie increase the % difference) you become more and more like a locked differential. Something else to note, is that the clutches can, and do, eventually wear out. This means that periodically (not that often) you do have to service them with new clutches. The other two only wear out if you manage to wear a gear down to nothing.
So why do people not chose the torsion differentials all the time? For the most part, price. A conventional limited slip is less than half the price. The cheapest torsional diff is over $400 for just the part, not installation. Spooled diffs are good for people that go off-roading or need to handle massive HP numbers (ie. full time dragsters). Off roaders like it because if you have one wheel in the air, you still get 100% power to the wheel on the ground, pretty much a must have for rock crawlers. In this case a torsional diff would not work as good, as it depends on that wheel being able to handle at least some force.
So now you know. Informed decisions make good decisions....this thing turned out good, I'm going to have to post this on GMN.