I have a 2010 F150 it has a 6in pro comp lift and 455 rear gears. I have 68000 miles on it and have a tranfer cas seal failure. With the 6in lift does the rear need to be tilted. I think is was a little early for the seal to fail.

 

I'm curious, too. I just installed a Rancho 4" lift, which uses the taller blocks under the leaf springs, and now my T-case is leaking at the driveshaft seal.

 

you might need to have a double-cardan style joint made in place of the single u-joint at the transfer case on the drive shaft. the single joint with the added angle of the drive shaft with the lift is not ideal because it puts that u-joint at too much of an angle. its probably putting enough of a strain on the tail shaft of the T-case to cause it to wear the seal premature. thats just my guess anyways.

double-cardan joint: