While I am confident we will neither resolve, nor reach a consensus, on this forum, we can provide some analysis summaries and allow others to make the decisions they feel best suited for them.
-The OEM 75W oils (US and EU specs appear to be identical) and other 75W80/90 oils are, indeed, quite different in anti-wear additives (OEM has way more) and viscosity. This might matter for 2 reasons. 1) If you plan to keep your truck well past 150-200k, I'd choose the optimal protection (also OEM specified) for longevity and minimum repair costs, knowing I'd be spending more money up front for 30k-40k mile interval changes. 2) If you have a GX, LX, or 4Runner Limited with the Torsen limited slip center differential, the antiwear properties may be much more critical for longevity than if you just run a standard 2 speed transfer case.
-Both Mobil 1 and Amsoil, 2 of the biggest names in higher end lubrication, have stated they do not have an alternative to offer. Possibly too expensive to customize a formulation with a relatively low sales volume, and they understand their liability if they claimed an existing product they had was a viable alternative. I am guessing they know it has unique (and expensive) properties as I'm sure they've analyzed it as well for competitive analysis purposes. It may not make for a compelling business case for them. If they were confident that their own offerings would be suitable alternatives, one might suspect they would recommend something they currently offer.
-You will find copious amounts of individuals who swear that standard 75W90 works just fine for them. And it may, indeed, work very well for them. The question is, for how many miles will it work just fine? There is not a definitive answer on that yet--controlled experiments to answer this are virtually impossible to run based on costs and cost of replication. Toyota specifies 75W90 for the rear and front diff, but something different for the transfer case...I would guess it is not for the sake of being difficult.
-Finally, I'm perplexed that the dealers do not carry their own specified 75W OEM oil. My local shop uses a mineral based 75W90 for all 3 differentials, probably because it is cheap and fewer part #s to stock and most customers would squirm if they had to pay the OEM 75W price for 2 qts. The dealers do need to stand behind their service and material selection for warranty purposes. And I am sure they do/would. However, when you look at the warranty period, even Toyota/Lexus extra care powertrain warranties do not extend past 125k miles. I'm sure even the cheapest gear lubes would protect well enough within this time period. How much of a risk do you want to take, and is it worth the fluid change cost savings for you? That question each of us must answer on our own. After all, if the OEM 75W oil was ubiquitous and priced the same as alternative 75W90 lubes, I think this debate would have ended years ago.
-I hope at least someone finds this summary helpful, even if only to generate your own thoughts and analyses.