18 speed trans issue a lot of play in the output shaft

So I bought a 18 spd Eaton out of a 09 peterbilt. Truck had 900k miles and the trans had no problems. I had a buddy rebuild the transmission replacing the top gear set and all other wear items. (He also rebuilt my 10 spd and it has worked flawlessly). Here's the issue- I just got the trans bolted up and I'm finishing up the job with the yolk and driveshaft install. I tighten the yolk on the the output shaft and I have a lot of up and down and side to side play in the output shaft, a full 360 degree worth of it and the nut is tight! Now there is very little or no in and out movement of the yolk (shaft) but probably 1/2-3/4 inch up and down at the U joint. The yolk, nut, and sensor ring are all correct, the nut is not bottomed out on the shaft, so I am figuring the is a problem with the shaft somewhere inside the rear box.. maybe a spacer or something that's been left out?? I have no idea. I forgot to look at the trans tag at my shop before posting this, so forgive me that I don't have the model number of the trans at the moment. Any ideas are greatly appreciated.