Keep losing my driveshaft

[Soxfan05]

I'll try and explin this the best tht I can.I have a 90 Jimmy with a 350 and a 4 inch lift.Acouple of months ago I had my transfercase rebuilt.To save a couple of bucks my cousin and I decided to take the tcase out ourselves and bring it to the shop to get rebuilt.After I got it back we put it back in.A couple of weeks later I was driving home I heard a big bang and I quit moving.I lost my driveshaft.I found the driveshaft and the yolk.The yolk was broken.I bought a new yolk and had my driveshaft rebalanced and put it back on with new u-joints.A couple of weeks later I was driving home again and I heard a bang and again quit moving.My driveshaft fell off again.This time I didn't find the yolk though.So I bought another new yolk.I don't want to put it back together though because I think I'm doing something wrong.Either I'm doing something wrong when I put the U-Joints on or I did something wrong when I put the transfercase back in.Both times I lost the driveshaft the straps were still on the rear axle along with the u-joint so I'm thinking the problem is with the front u-joint that connects the yolk to the driveshaft.Any suggestions?