driveshaft keeps fallin out

I got a 92 1500 4Wd..it has a 6 inch susp lift and 35's and hasnt giving me any problems since i got it a year ago..but 2 weeks ago the driveshaft fell out and i replaced the u joints and thought everything was ok..well a week ago it fell out again..i replaced the u joints and got 15 feet outta my drive way..replaced them again and the same thing happened...No fluid is leaking out of the rear end..it has leaked probably a quart or two of tranny fluid whenever it falls out but thats bout it..im thinking it might be the actual Yoke on the rear end but i dont know..i dont really know anything on rear ends..Anybody have any suggestions on my next step....

Thanks
----Chris

Doesn't sound like the pinion angle was done properly when the lift was done, and now the trans yoke bushings are worn out.
Which driveshaft is falling out ? The front one or the rear one ?