Rear Transmission seal?

I replaced my U-joints and put in a new rear transmission seal yesterday. Everything seemed to go great until I got my truck back to my house and parked in the driveway, jumped out and looked under my truck and noticed a ton of tranny fluid pouring out where the drive shaft enters the transmission. If I park it on a level surface no fluid leaks. As far as I can tell everything was put back together right and I have the right seal for this specific transmission. The new seal and the old seal looked exactly alike. Any one have any thoughts as to why this is happening or what else I can do to solve this problem?

did you by ant chance overfill fluid? also, the seals are easilly ripped by the smallest bur or dirt.went through a couple before i found a small nick in shaft.

did you put grease on the yoke where the seal rides?