To me, this is a complicated answer because there are several Dana 70s (Dana 70, Dana 70U and Dana 70HD). There are three 10.25 Sterlings, the 5300# GAWR semi-float, the 6250# GAWR (Gross Axle Weight) full float and the fairly uncommon 10.25 DRW version (7400 or 8250 GAWR... the only differences between the weight ratings is the tubing wall thickness). The later 10.50 Sterling adds a few more variations but has the metic wheel bolt pattern. The 6250 GAWR Sterling is by leaps and bounds the most common.
Your '84 F350 is listed as having a Dana 70-2U originally, with a 6300# GAWR (Gross Axle Weight Rating... the axle pros pronounce that "Gowwer" rhyming with "Sour"). The Dana 70U first appeared in '84. Prior to that, F350 SRW had a mix of Dana 61s and Dana 70s.. depending on the axle ratio used (See "Original Dana 70" below)
From early '85 and earlier, you mostly found a Dana 61 full floater under F250 HDs and some F350s. It came as a semi-float for the 6600 GVW F250s and as a full floater in the F250 HD (4x4 and 4x2 in. 8600# GVW and larger). It's an oddball, ****** cousin of the Dana 60 that was redesigned so it could carry taller gears down to 3.07:1. It has a 9.75 inch ring gear and some of them have very odd pinion offsets so you can have trouble swapping gears. It has the same GAWR as the later Sterling. In fact, the Sterling was designed to replace this axle in early '85.
The original Dana 70... It shows up in some for applications prior to '85 but only where a ratio of 3.73 & lower was used. 3.73:1 is the lowest ratio that fits in a "regular" Dana 70. When the fuel crunch hit, they wanted taller ratios and Dana had to redesign the housings to fit taller gears and that's where the D70U line came in. Same goes for the D61 vs the original D60.
As far as I can see in my Data Books and Dana manuals, the only Dana 70 offered in F350 SRWs of this era was the Dana 70-1U or Dana 70-2U (generally considered in the D70U family... the "-1" indicates a version change). The "U" stands for "undersized pinion bearings. They aren't terrible axles but they have small pinion bearings and weird ring and pinion offsets that can make them tricky to work on and especially, to make a gear change on. Many people don't like the D70U in hard tow application because of the undersized pinion bearings. If something fails in hard towing use, that's what will go.
Dana 70HD... wasn't offered in Fords of our era except as a DRW. There were some rare SRW Dana 70HDs in GM trucks. This axle has very thick 4-inch diameter tubes and larger carrier bearings. With a larger ring gear, the D70HD morphed into the later Dana 80. Many people think the D70HD is a better axle than the D80.. but I won't bore you with the details.
So, after all this... I say go Sterling. It's plentiful, easy to ID and easy to work on with no oddball pinion offsets or anything to make them tricky. They can come in factory ratios of 3.07, 3.31, 3.73, 4.10, 4.30 4.56 (more ratios available from the aftermarket). Ford used a higher grade of steel in the axle shafts than Dana, SAE 1050 high carbon steel vs Dana's 1035 or 1040... so Ford's axle shafts are about 20 percent stronger. The Sterlings used 3.50 inch axle tubes, where the D61 and some D70s used 3.1-inch tubes. Most Dana 70s/70U use 3.56 inch tubes and the D70HD used 4 inch. Most 6250# Sterlings have .258 wall tubing, with the DRW versions using .448. The Dana D70Us listed for an '84 F350 SRW truck, 6300 GAWR, shows .410 wall tubing, where in the Sterling listed for the '85 model years, 6250 GAWR is listed with .258 wall for the F250 HD but .448 for the F350. I have verified the wall thickness of .258 in F250HDs but have never had the chance to do so on a SRW F350. It seems strange they would have .448 tubing on an axle still only rated for 6250#. Still, I would suggest trying to find a Sterling From an F350 vs an F250 on the off chance it has thicker wall tubing.
In general, Dana has the edge on axle tube thickness but supposedly, and I have not been able to verify this, Ford used a higher grade of steel in the HREW tubing on the Sterling than Dana used.
due to lack of oil from a long term pinion leak. The other oddity is that it had an strange carrier and ring & pinion combo that was giving the tech fits. Turns out it had a carrier from a high gear ratio with an extra-thick 4.30:1 ring gear. The extra thick gear was more expensive than a new standard gear set plus the carrier that fit, so that's the way they went.
