You raise a good point, as I said I’m new to Toyotas. So with my t-case the front driveshaft is not spinning when I’m going down the road in 2wd? If with a LS it would be spin all the time that would create a slew of new issues and yes a lot more ware. And if that is the case that would also explain why locking hubs would be my best option if I wanted to use any traction aid that can’t be turned on or off.
Without manual hubs, your wheels are permanently tied to your CV axles, so when the wheels are spinning the axles are spinning. The ADD splits one of those CV axles when you're in 2WD so that the wheels aren't connected. One wheel (on the side without the ADD actuator) is always spinning the diff, but because it's a differential, it mostly spins free and very little of that force is transmitted to the front driveshaft, which also spins freely because it's disengaged at the tcase.
If you put an LSD in the front differential, then it's going to do what it's supposed to do and try to match the speed between the CV axles. The ADD side is still going to be disconnected in 2WD so you won't feel a difference in the wheel, but there's going to be much more force transmitted to the front driveshaft, which is what Singtoe is talking about. That's a slight mileage hit and a little bit of increased wear on those drivetrain parts - not a big deal.
When you DO put it in 4WD, you will get slightly more traction on level ground, which is a good thing. You will also get slightly harder steering, much less RTC, and a fair amount of understeer on slippy surfaces like ice or sand. If you articulate and unload a wheel, the LSD will not be strong enough to keep power to the loaded wheel like a locker would - you'll still spin just like you had an open diff. So really, the only place the front LSD is going to make a difference in your truck's ability is on steep unarticulated climbs, or deep but level mud/sand/snow situations. If those drawbacks are worth the slightly added ability in those specific situations to you, go for it.
My concern is that if Singtoe is right and you have to disable the ADD in order to install the LSD, then it's going to be working
all the time, 2WD or 4WD won't matter. And I'd call that a Bad Thing for a street truck. You could install manual hubs, which have their own set of benefits, but by the time you buy the LSD, install it, and then convert to manual hubs, you'll have spent almost as much as if you'd just bought an airlocker and compressor in the first place.
