Center the kuckle by adding / subtracting shims on bottom trunion cap,
shims on upper set knuckle bearing preload.
The knuckle would have to be way off to ruin a seal in 20 feet of movement . . .that's like 4 or 5 revolutions of a tire.
R U putting seal in correctly ( well seated in seal area, pocket for seal clean of prior seal, etc?), even wrong side out shouldn't cause 'immediate' failure. Once seal is set, reach in with a finger & try to gently remove....should not come out easily (with one finger)....if it does, either seal is wrong or not set properly.
Using correct seal ? It might fit in the axle housing (seal pocket) but be too big an ID to seal on axle shaft ? slide seal over axle shaft to check for fit....
Oh, is the 'leaker' on driver's side (long axle side)? you could be dislodging seal as you attempt ,

, to get axle into diff....
Lastly, does long part of axle fit snugly in birf joint to short side of axle ? ( is one half way out of alignment with other half....broken birf ball, rusted , jambed off center)
I'm presuming axle is not bent....
Good luck
Pete