Question about Transmission Output Shaft Seal

First post here guys and I am sorry if I am putting this in the wrong place. I took my '05 PreRunner with 147,000 miles on her in to get PM services done and they said my Output Shaft Seal was leaking and needed to be replaced...$400! So I told them to go ahead and do it, I am a pretty handy guy and I would have done it myself but I just don't have the time. While I was talking to the mechanic which he was foreign and hard to understand he told me that I need to watch that seal to see if it starts leaking again...because he said that the angle of the drive shaft is causing pressure to be put on that Shaft Seal basically because the truck is made to carry a load which with a load will press the suspension down and therefore the driveshaft more parallel to the ground thus no pressure on the Shaft Seal. He then told me that if it starts leaking again I will probably need a whole new drive shaft...$1500! So I asked him how much more milage I could get out of that shaft before replacing it and he said 20,000 - 30,000 - 40,000 it is really hard to tell. So, I guess I am kinda miffed about the driveshaft deal and was just wondering if any of yall have heard of this problem on this particular driveshaft and any information that I could gain from yall on this.