So, what could go wrong flat towing my '99 4Runner behind a motorhome? I mean, beyond towing a ... jeep.

Burn up the output bearings on the transfer case I believe.
Far as I know these have to be trailer towed, dolly with rear driveline removed, or flat tow with both drive lines removed.

Read the owner's manual on this subject.
I have seen prop shaft disconnect devices installed on the 3rd gen, but don't have any user experiences to report.

I always tow my 4 runner on my Carson utility trailer behind my motorhome. It saves wear and tear on the 4 runner and the trailer just needs occasional tires and wheel bearing greased. If you need a tow truck to tow you these things require a flat bed and can't be towed by a uhaul dolly either.

This is the answer right here. Can also ruin the transmission if left in 2WD with the driveshaft still connected.

With the rear driveshaft disconnected (manual or with some device), it can be flat-towed in 2WD.

-Charlie

Does this mean it can be flat towed with a manual transmission?

with driveshaft disconnected

What breaks if you don't disconnect the driveshaft? Specifically? The book says tow dolly with xfer in 2wd and xmission in neutral. So what's the difference? Any actual personal experience with it? I have a Sprinter based RV and adding a trailer overloads it. A tow dolly is my current option. I know the old FJ4x had steering geometry problems with flat towing, but I would think Toyota would have been able to fix those problems with the IFS. I flat towed my '85 4Runner 900 miles one day with no problem. I can see the problem with an automatic, but I don't see it with a manual xmission. I also just updated my '99 4Runner with a '21 Taco 6 speed, and thinking tow dolly anyway with that as well. The book for the '21 says not to flat tow is all - "not designed for it."

What book are you referring to? The owner's manual for your 4Runner?

Doesn't the transfer case have a oil pump in it?
I would think the reason why for removing the rear driveshaft while being towed is to prevent the rear output bearing (among other things) in the transfer case from turning without oil.

Yes, the issue is the transfer case oil pump, which is on the *input* side of the transfer case. If the transfer case is in neutral, it isn't lubricating the transfer case (=boom)

Same deal with the manual transmission and neutral. The oiling doesn't work correctly with only the output shaft spinning.

-Charlie

i dollied mine from texas and just disconnected the rear driveshaft. Takes a couple minutes and better safe than sorry. Considered flat tow because of weight as i was towing with a frontier but read a thread similar to this one and didnt seem worth the test.

One thing i did run into was that the standard uhaul tires straps didnt quite fit over 33s so i had to unload and go buy a set from Northern Tool before leaving.

For anyone who wants pictures of how bad flat towing can go, check out this article.

I saw the video on a facebook mechanic's page and it's impressive

All kinds of bad.