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I tend to like the tread design on the IROKs for sentimental reasons. They remind me of the old directional Firestone Super All-Tractions I used to run.

I'm having trouble deciding between the I-810 (13.5x39.5-16.5) and the I-815 (14x42-16.5).
Any thoughts about choice regarding clearance, steering, spring loading, etc.?

The 39's will probably steer a little easier than the 42's and clearance would probably be a little better over all as well.

I prefer the SX over the Irok, simply due to the larger outer lug makes it easier to climb out of ruts...

True about the SX. If they had a directional tread, I would be more interested in them.

I heard recently that the TSL Bogger is coming out in a 40-41" size with a fairly narrow tread. Nothing beats a bogger for directional traction. Other than maybe an ag. tire...

Thanks for the tip. I like tall tires with narrow treads.
Much appreciated,
Jim

42" will have clearance issues with stock suspension, even the 39's will rub.

Is the rub at the top, due to articulation?
Or at the front and rear, due to turning?

In the front you will run at the top due to full articulation, and on the rear inner fender when turning while articulating.

In the rear, the side of the tire will run the inner fender on the bed side at full articulation.

In the rear, is it dependent on backspacing?

P.S.
Backspacing on the rims I was planning to use is 4.43 inches.

Yes, rubbing at all point will be affected by back spacing. I'm running 17x10's with a true 7.75" backspacing (~3/4" more than the Budd's).

IF you go with that little back spacing get ready for a lot of top fender rubbing and some odd steering effects.

Sounds like a spring lift would stop the top fender rubbing. (not a problem on the rear of the 708 -- they don't have rear fenders)
On the downside, I tend to run at very high off-road speeds, so steering is important.
What odd steering effects are you seeing?

The tire contact patch on the M37, has the center line of the steering king pins about 2/3 in from the outside of the tire. Moving to that small of a back spacing and the taller tires will move the steering center line suck that it would likely not be in any part of the tires contact patch to the ground. Resulting in a larger Scrub Radius which will make it steer easier as the wheel can roll when turning, but I'd expect more off road hard feedback into the steering wheel, and any differences in braking from side to side to cause the truck to pull hard.

Yeah, it already does that (for slightly different reasons).
I've had ruts grab the front tires and yank the steering wheel out of my hands several times.
Only caused it to turn over a couple of times though. The one downside of the 37's is that they are easy to flip.

IMS what Will is refering to is called the "Ackerman angle"...