This isn't always true either. 4WD can mean that you send equal amounts of power to each axle, but at that point there are things that can change the amount of power going to each wheel within the center differential casing. If you have a limited slip differential (why some 4WD vehicles can drive perfectly fine on pavement) it'll basically send a 50/50 split (lock the axles together when going straight) but allow the axles to turn at different rates when making turns (otherwise bad things would happen). Some cheaper 4WD vehicles (and many 2WD vehicles) have open differentials. Open differentials send the power to the path of least resistance (bad for traction, and also why if you're in gravel the loose wheel usually just spins while the other one doesn't). There's also a fully locking differential that literally locks the axles no matter what. These cannot be used on pavement for obvious reasons (well, think about turning a corner, the outside wheel will have to turn faster than the inside wheel). AWD vehicles have a center diff typically and also often employ limited slip differentials (but sometimes again, they have open and/or auto-locking diffs...ARB made air locking diffs at one point, not sure if they still do) at each axle as well which enables a mechanical distribution of power. Some newer vehicles have replaced the axle differential gears with anti-lock breaking systems that provide resistance to open diffs to regulate wheel spin (artificially add resistance to the spinning wheel).....sorry, before I became a computer geek I built motors and off-roaded in my 81 CJ-5, after that I drag raced for fun on the weekends at the local track. I had a limited slip in the mustang I raced and a fully locking one in my Jeep. Jeeps (and older 4WD) are interesting b/c many come with what are called locking hubs ( https://en.wikipedia.org/wiki/Locking_hubs ) which are not all that common today. Even if you do find them, they're usually auto-engaging vs. the ones you have to get out an manually turn to engage the front drive shaft.
I once had a Jeep Grand Cherokee that had a "full time 4WD" system. Basically you could choose between 4WD low (traditional 4WD with front and rear diffs locked to each other, in a low gearing for massive torque), 4WD high (traditional 4WD as above, in normal gearing), 2WD (front diff unlocked and spinning freely as if it were a regular 2WD car) and full time 4WD (front wheels would automatically lock together only if the system detected slippage in the rear, and would receive about 35% of the engine's total power output).
The funny thing was, even though you were supposedly able to drive in full time 4WD on dry pavement, it would sometimes engage the front diff in parking lots and when taking tight turns at low speed, causing the front wheels to hop. I ended up leaving it in 2WD unless I was on a dirt road or it was raining, figuring full time 4WD was a marketing gimmick to compete with Subarus.
From what I remember when I had to replace the transfer case a year after I bought it (it was a 1996 that I bought in 2006 and the previous owner abused it), it was more or less a "traditional" Jeep 4WD setup but depended on the transfer case to keep the front and rear axles independent until slippage was detected. I think it was pretty much fully mechanical, except for the auto locking front hubs. I believe Jeep's marketing term for it was Select Trac or something like that.
I think the issue was with the auto locking hubs; when it malfunctioned it would lock the front hubs together and in turn the transfer case would lock the front and rear diffs together, causing the wheel hop. This defect probably contributed to the death of the original transfer case, now that I think about it.
