1) low quality seals
2) groove on yoke where the lip rides
3) no sealant between seal and housing
4) vent plugged allowing pressure to build up in rear end when hot
5) worn pinion bearing allowing yoke to wobble when spinning
Actually, there is a bit more information I left out of the first post because I didn't think it relevant but several of the causes listed above were considered with the first seal replacement which I did before taking it to a shop.
I am a bit of a gearhead and have been wrenching on a variety of machines since I started handing my Dad tools when he was making vehicle repairs back in the '60s. I have replace a variety of seals in a variety of machines from Caterpillar heavy equipment, to Chevy and Ford vehicles, a variety of off and on road motorcycles, etc., so I felt quite competent to replace this pinion seal.
As was pointed out to me before taking on the repair, the seals are a bit of a pain to install so when the first seal I replaced started leaking after, I am guessing now, less than 5K miles, I figured I must have screwed something up. However, I did not use an OEM seal nor did I apply any sealant between the outside of the seal and the housing bore but I did check the yoke for a groove and found none and I did check the vent and found it open. I doubt that the pinion bearing is worn enough to allow the yoke to wobble at 149K miles but it is something to check out.
That is when I decided to take it to a shop where someone that had replaced more than one of these seals might be more successful. I am pretty sure the tech did not use an OEM on either replacement as his shop is associated with Advanced Auto and that is his parts supplier. I do believe he applied sealant (not sure which one) to the outside of the seal and housing bore as the second seal he replaced does leak but does not appear to be leaking as bad which is why I suspected that is where the seal is now leaking but that is speculation on my part.
So, I will pick up an OEM seal as has been suggested and review all of the above referenced potential sources of the problem to the tech when I return the vehicle to him. He did ask that I bring the vehicle back in after a couple of weeks so I am pretty sure he intends to solve the problem one way or another.
Thanks for the suggestions.