I became a RAM guy in Aug 2010, so I guess I am wrong on when they got a full time 4wd system.

As for the definitions, you are getting hung up on thinking named based on how they work, which is not correct. They are named based on WHEN they can be used. What you are referring to by being able to go back to 2wd is the difference between AWD and 4wd. AWD has no operator intervention, only 1 mode. 4wd has a manual transfer case of some sort. What is on it, varies from vehicle to vehicle, and whether or not it can go to 2wd or not is irrelevant.

This is basically what we have. See how Jeep clearly defines a "part time" and "full time" 4wd? Also, there is a 2wd mode, so that means your "full time" theory doesn't work.

The NV242 provides full time 2-wheel, or 4-wheel drive operation. A differential in the transfer case is used to control torque transfer to the front and rear axles. A low range gear provides increased low speed torque capability for off road operation. The low range provides a 2.72:1 reduction ratio
The 2WD and 4x4 full time ranges can be used at any time and on any road surface. The 4x4 part-time and 4 Lo ranges are for off road use only. The only time these ranges can be used on hard surface roads, is when the surface is covered with snow and ice.
The geartrain is mounted in two aluminum case halves attached with bolts. The mainshaft front and rear bearings are mounted in aluminum retainer housings bolted to the case halves. Two versions of the NV242 are used in the WJ vehicles, NV242LD and NV242HD. The two transfer cases can be distinguished from one another by the rear output shaft retainer. The NV242LD uses a rubber boot to cover the rear output shaft, while the NV242HD uses a cast aluminum housing.
Operating Modes:
2WD (Sends all power to rear wheels)
4WD High, full-time (Sends power to all wheels)
4WD High, part-time (Locks both front and rear drive shafts for increased traction)
NEUTRAL (Allows vehicle to be towed)
4WD Low, part-time (Doubles the transfer case gear ratio for severe road conditions and low-speed pulling power)

Part time = don't use all the time, only when needed and conditions allow for it, hence "part time 4wd"

Full time = can be used whenever on any road conditions, hence "full time 4wd"

Check out this Jeep wiki, and read up on the different transfer cases they use. All the definitions are there.

http://en.wikipedia.org/wiki/Jeep_fo...-drive_systems

 

lol....Gutty the last thing I need is a lesson Jeeps. A CJ was my first vehicle and I've owned 7-8 Jeeps since. Still have a CJ, which Wiki doesn't even describe those systems. I guess it wasn't a Jeepbrand system. We always referred to any truck that had the ability to select 2wd as being a 4wd-PT and the ones that you couldn't as 4wd-FT regardless if the FT had a lock in it or not. A simple 4H and 4L was it. Funny, Ram even describes in the literature their 4wd as a "4wd-PT" system regardless of xfer case. As such, it describes the whole truck, but I will concede sir, you are correct in the naming of the individual systems. Although I will still call our trucks a 4wd-PT though because saying it's a 4wd-FT and PT is too much. I guess, in a way, what we called "part-time" is the same definition you are using because you could only engage it part of the time.

 

Deal.

Regarding the other parts. For the purpose of answering the OPs question of when to use each setting. the true definitions (not how one likes to call them for their own description) will be most helpful. So help the OP decide...

4wd Auto = Full time, use it whenever you want.

4wd LOCK = Part time, use it when conditions allow (or you *may* risk damage if used on dry roads)

Remember the full time/part time is NOT a description of how the vehicles transfer case works, but rather when you can use each setting. The whole 2wd thing you are hung up on, is more AWD vs 4wd (no operator control vs manual transfer case) then it is part vs full time.