The difference lies in "part time" vice "full time" operation.
In PT4WD, the front and rear axles are locked together in, I believe, a 52/48 front-biased torque split (I've been told 50/50 - practical, but the front-biased split makes a little more sense, since it allows you to pull out of situations. I've yet to verify this.) This is also borne out by the fact that you can't run PT4WD in situations where you already have good traction for extended periods - pictures have been posted here of grenaded transfer cases from doing just that. In the absence of confirmation, I'm going to stick with the 52/48 idea - I've heard it more than the 50/50, and anecdotal evidence points that way as well.
For FT4WD, it's a differentiated base 50/50 torque split - generally even, but can bias toward either end as required for changing conditions. The 50/50 differentiated split is why you can run FT4WD with good traction with minimal ill effects (theoretically, it's a step short of AWD. Practically? If you don't need it, you really shouldn't run it - but if you think a need for it is coming up, you can shift into it before you need it. This saves you thinking about it later.)
Since rain does not impact traction as much as snow does (rain does not fill up the valleys in the tyre tread like snow will, and can be "pushed out of the way" by tread blocks separated by either properly-designed or just plain wide grooves,) the FT4WD can be run more effectively in the rain and wet because it doesn't have as much trouble with "getting a grip" as PT4WD with the locked (and biased, I'm sure) torque split. However, the "locked differential" you are referring to is going to be found in the PT4WD case, not the FT4WD case. However, the NP242 can be locked into PT4WD mode - and in both the NP231 and NP242, the "low range mode" is locked/PT4WD.
Case lengths are similar (to within a quarter-inch or so, by visual estimation,) and mounting patterns are identical. Input gear length may be different - pre-1991 NP242 are going to be 21-spline, and 23-spline after that. Find them in XJ/MJ "Limited" versions. I believe the NP242D can also be found in Dakota and F/S pickup, but I've yet to verify this as well (workin' on it!)
The NP231 is rather more common, with the NP231J being what we have. The NP231D and NP231C can be found in Chevvy and Dodge trucks (respectively,) and there is also an NP231HD variant (Heavy Duty, obviously,) and NP231DHD (Dodge Heavy Duty, I believe.) The primary differences there are the sustained torque throughput capacities - the cases are externally the same, and use the same mounting pattern (which is, in its turn, derived from the old Dana 300 case. This pattern is now standard for pretty much everyone.)
I am not sure if the NP231 input set will mate with the NP242 geartrain - but I believe Novak offers input gears for both cases, made new. I do know that there was a change to the cut of the input gears for the NP231 ca. 1994 - crossing that line by putting an "early" input gear into a "late" case (or vice versa) will see you draining much of your input gear at the first fluid change. I would think that the NP242 would be similar - I suspect the change was made to improve the pressure angle on the gear teeth (to improve longevity, I hope. I've yet to find out about that as well - I may end up going to New Jersey to sort this out.)
Did I cover everything you wanted to know?
