A couple guys here have touched on this already but I'll spell it out: 4.10's will tow more and easier than the 3.54's due to torque multiplication. Downside is rpm's go up - that's where the 3.54's come in. It's a give and take thing. Personally I like the gear vendors idea, best of both worlds.
But that brings up another issue - Robert Rausch said it pretty good - torque curve's long gone by 2000 rpm. Anything past peak torque rpm while towing is wasting fuel. The problem is forcing the engine rpm's down that low while working the engine that hard makes the EGT's rise high enough to easily melt pistons. With my mods, I pulled a 40" 5th wheel into a healthy headwind at 50mph two weeks ago. Here's what it looked like with my 3.54's:
50mph
1500rpm
1250* on the EGT's.
(I should mention I had a cardboard in front of the rads (-18*c) which means there was no air-air cooling)
Any faster and she really spiked. A guy really needs a twin turbo setup to move a high enough volume of air for sufficient cylinder cooling. This was on the flats - one little hill and I would have had to drop a gear and crawl. Just not enough air flow with the stock turbo. It's all a chain reaction, change one thing and something else will suffer.
If you run a gear vendors, you can haul along all day at low rpm's, but when the going gets tough you just drop the splitter, gain some RPM's and keep on chuggin... That's my

 

I talked to Gear Venders about a gear vender OD I found. The tech I was talking to said that I would not be able run both my transmission in OD and the gear vender in OD if I was hauling my camper or pulling a trailer. There would be no problem running it that way empty he said. My thought then is what good is a gear vender OD if you can't use it when hauling something.

So, back to square one, go slower or get there faster with less money or change my gearing.

where is that two speed rear end

 

You cannot tow anything with that amount of double OD. OD is the weakest gear in a transmission, double OD is bound to break with any sort of weight behind it and abundance of torque going through it. The trans OD would probably break before the GV OD though since it is the weaker of the two.

You gain nicely when unloaded running double OD though. If you tow light (less than 4k) you could do the double OD i am sure.

If you tow in direct gear (trans) and GV OD, it is stronger than the OD of your trans alone.

 

Actually, the direct clutch in the trans is the weakest link (outside of tq lockup clutch if it's a single disk). There are kits available to add more clutches to the direct drum.

Gotta remember, when the engine is geared down, Torque increases (horsepower does not) and visa versa. For example, in OD, it's straight engine torque that's going through the OD clutch where as in 2nd gear for example, engine torque is multiplied by whatever ratio 2nd is and that increased torque is what the direct clutch sees...

OD's DO fail for sure, but the direct clutch usually goes first. There was a guy on here (or maybe it was a different forum) that proved it by doing the math on all the clutches according to stock # of plates, surface area and apply pressure, but I can't find that thread now.

Case in point: When I did my trans, TQ Lockup clutch was screwed, 1st clutch was pretty worn, Direct was completely toast but both clutches in the OD were perfect, so because my master rebuild kit was sucky (read: cheap), I left the old clutches in the OD 'cause they were better than the new ones! (more engaging lugs on the hub, and still lots of material)

 

Sorry, in an auto yes. I was speaking more in terms of a std trans, that's what i think of when planning to do any towing.

Guess i should have paid more attention the the OP details.

 

No problem, I just like to talk...