If you have a new yoke and you're still eating seals, your pinion bearings may be loose. If they are loose, then the pinion will move around to the point where no seal will last. A second possibility is that the gear lube is running up the splines and coming out from under the pinion nut. Don't laugh. It's very possible and fairly common. Most factory pinion nut washers have a rubber seal built into the center to seal the splines. Silicone or gasket makes will work if your rubber is shot... just make sure it's very clean first.
Also, torque for the pinion nut is 220-280 ft/lbs. While I agree that the proper way is to use a torque wrench, most 1/2" impacts will have enough working torque to hit this range and I don't think that that's the major problem. They may still be installing incorrectly and damaging the seal, or it could be leaking from between the housing and the outside of the seal, or you could have the wrong yoke. There are a lot of things that could be wrong and unless you're up-to-speed on differentials and you watch the process, it's really hard to know where to start.
I would trust the install process for now. Let's look for something they may be missing. You've already checked the vent, added a new yoke, several seals, and several installs.
Let's check for loose pinion bearings: Chock wheels and set parking brake. Put truck in neutral. It's best to take the driveshaft loose from the rear end, but not 100% necessary. Grab the yoke and try to move it around. There should only be radial movement. In other words, you should be able to twist it a few degrees each direction due to normal backlash between the ring gear and pinion gear. However, you should NOT be able to wiggle it around. If you can move it around, then you'll need to reshim the pinion bearings for proper preload.
Dana 70s don't use a crush sleeve between the pinion bearings to set preload. They use shims between the outer pinion bearing (closest to the yoke) and a shoulder on the pinion shaft. If your truck has a lot of miles, or if the pinion seal leaked for a while before you caught it and it ran with low oil, then you could have excessive wear in the pinion bearings. Don't worry, though. Re-shimming the pinion bearing preload is not a difficult job for a competent shadetree or a decent shop mechanic.
The preload spec for new and used pinion bearings is different due to initial break in. New bearings should be set a little tighter so that when they break in, they will not be loose. Used bearings are already broken in and will usually run exactly how you set them. Test bearing preload with the differential removed and the pinion bearings oiled.
AXLE SPECIFICATIONS for D70 taken from 2001 service manual:
DESCRIPTION SPECIFICATION
Axle Ratio 3. 55, 4. 10
Ring Gear Diameter 267 mm (10. 50 in. )
Ring Gear Backlash 0. 12-0. 20 mm (0. 005-0. 008 in. )
Pinion Gear Standard. Depth 136. 53 mm (5. 375 in. )
Pinion Bearing Preload - Original Bearings 1-2 N·m (10-20 in. lbs. )
Pinion Bearing Preload - New Bearings 2. 3-5. 1 N·m (20-45 in. lbs. )
DESCRIPTION Ft. Lbs.
Fill Hole Plug 25
Differential Cover Bolts 35
Bearing Cap Bolts 80
Ring Gear Bolt 130
Pinion Nut 220-280
Axle Shaft Bolts 95
Hub Bearing Nut 120-140
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