Thanks for the input growe58
Previously I would have said this wasn't the problem because the u-joint went together nice and snug. But after replacing the wheel bearings and the trunnion bushings I was running out of things to try so I decided to replace the u-joint again thinking I may have screwed something up the first time. Once again everything seemed to go back together without any problems so I took it out for a test ride. Well there was no noise for the first 500 ft but after taking a left hand turn the noise came back only this time the "knocking" was more of a "clicking" and it only occurred once per revolution. This latest symptom sure seems to fall in line with your diagnoses.
Could you check out this video and tell me if the "clicking" sounds like the problem you've described.
The good news is I think I can remove the rear axle in less than 10 minutes now and if this fixes the problem I'll have a fully refurbished rear wheel (and only a few hunderd more grey hairs).