[MacGyver]

Sounds like you took the first logical approach, replacing the yoke was a good measure. I can't answer the question of seal type but assuming it is correct I have a few Questions.

1.How many miles on the first seal?

2.How many miles on the 2nd seal?

1. Have you checked the vertical play in the yoke. In other words grab the yoke and move it up and down. Excessive play won't allow even the best correct seal to work. This would indicate a worn tail-shaft bushing/bearing.

2. How close is the yoke from bottoming into the end of the tail-shaft when the rear suspension travels. Too long of drive-line or loose rear suspension components can cause the drive-line to bottom in the trans damaging the seal and possibly more.

3. One thing I failed to take into account, Is the rear suspension independent or straight axle? This would obviously effect the factors on question 2.

To check items 2 & 3 the vehicle needs to be on a ramp hoist, a pit or the rear axle needs to be supported by jack stands to simulate proper rear suspension load and drive line position.

Good Luck, Matt