What you report is different from what I was told, but I have the Dana 80 (manual trans) while you have the Dana 70 (auto). On the Dana 80 I understand that the 4. 10 is installed on a large carrier and the 3. 54 on a small carrier. I was told this when asking about limited slip differentials and gear options from a rear-end shop. I would get a second opinion on this before buying any gears as a new carrier will boost your gear-change costs way up.
If I had the auto, with its high overdrive ratio (0. 69), and I had my choice of rear-end ratios, I would not pick the 3. 54 as I think it's a little too tall. I would go with the 3. 72 (think that's the Dana number) as the tallest, and in fact would have used that ratio on my truck (with manual) if it was a factory option. The 3. 54 is quite tall a ratio for the 24 valve engine, especially with the auto trans. Note however that that's how I ordered my truck, and I am happy with it, but would have gotten 3. 72 if it had been offered.
Fuel mileage savings will depend how you drive. If you drive empty, and over 70 mph you should see some savings with a taller gear. If you drive at 60 to 65 mph your mileage will probably not change, even though you are at lower engine RPM. In any case, any savings in fuel mileage would be slight and it might take more miles driven to recover your gear change cost than you plan to put on your truck.
Check the previous issue of the TDR that has the graph with the ISB engine Brake Specific Efficency (right term?) shown. Best efficency is at about 2,000 RPM and does not change much between 1,800 and 2,300 RPM. If you currently cruise with the engine in that RPM range a gear change is probabily a waste of time and money.
To figure your new RPM at a constant cruise speed, multiply the RPM by a ratio of the gear ratio. For example if you are currently at 2300 RPM at 70 MPH your new RPM would be 2300 x 3. 54/4. 10 = 1986 RPM (at 70 MPH with new gears).
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Just got my 2001 2500 QC, SB, 4x4, ETH 6-sp, 3. 54 LSD, SLT++ (everything but those lights on top of the roof), White over Silver, Tan leather. Picked it up 8/31/00.