Don't know for sure, but worth looking into. Does your Truck by chance have a hangar Bearing?
If it is U-joints take the bad one to a good parts store where the guy knows what he is actually talking about.
If you have a good parts man he will get you one, they are mostly a universal fit.
Example u-joints from a (350 Chevy 1970s era) will fit a 48-50 Ford 1/2 ton.
Please excusr the Chevy, but just trying to get a point across.
I would further venture to say that the ones from your 3/4 ton Ford would fit, worth a look see.
All you need is a caliper for measuring and bingo your in business.

 

I was able to get the right U-joint today at Berg's Auto store we just looked up by measuring it and came up with the identical part and brand. the U-joint did not fix the problem though. I think it is the pinion bearing I an was also wrong about it moving back and forth it moves in all direction if i turn the drive shaft forward and reverse it apears to move in and out 1/4", could this be as simple as re torquing the pinion bolt.

 

The pinion should not move like your stating and that may be the reason for the worn out pinion seal. The seal needs to be replaced anyways which requires the pinion to come off. I've rebuilt a few rear end pumpkins and they are a pain. I would recommend that you pull the rear gears and take them to a rearend shop and have the rear end gone through and checked. They charge about $150 to completely take yours apart and put it together correctly. minus whatever is worn or needing replacing. Well worth the money, trust me. It will eliminate a lot of your problems down the line. Don't try retorqueing the pinion because that could make it worse!

 

No as I had seen in your earlier post and pic the cotter key was still in the pinion so the nut could not have loosened up.
I think you are off to some real crud now, because with all the slack the pinion bearings are shot and the knocking noise suggest a chipped or broken tooth on the ring gear because of the duration of knock.
If the rear end is what I think it is that comes apart in the middle with a right and left tube and housing you could first try removing the bolts around the pinion and pull the pinion and front housing out of the rear axle, from there gears can be inspected enough to tell whether you can put new pinion bearings in or will have to do a full blown overhaul or replace.
If the pinion bearings are that bad I wonder what the side bearings would be like. Good luck.