"Which end did the cap come off on? I had a cup come out on the pinion shaft joint on the starting line It ended up that I had spread the new Spicer forged steel rear pinion yoke apart It didn't look like anything was wrong to start with, once I tried to install a new U joint I found that one cup on the pinion yoke a tiny bit loose Just enough to let the interior keeper pop off"

Front u-joint right behind the trans is the one that lost a cap. I'm guessing the keeper snaped off when I got out of the throttle and that's when it all started coming apart.