I have a 1999 blazer 4x4 200k miles I started to leave work the truck came to an abrupt stop with a loud noise and would not move in drive, put in park then reverse it drove a few feet backward then put in drive and drove home but with a loud thump thump thump noise that got progressively quieter I am currently replacing the back u joint. Am I on the right track?

 

Brakes?

 

error_401 is correct. The rear brake calipers on the 4x4's freeze up quite often. It sounds like you might have worn the pads down until what was left of the lining came off and got stuck. When you backed it up, it probably broke in to pieces and fell out.
The best was to fix them is to replace both completely including pads and rotors. Take the 2 15 mm bolts out of the mounting brackets and disconnect the brake line at the banjo bolt. Then calipers and rotors. Might have to hammer the calipers off if they are really stuck and won't back off.
Rockauto has NEW not reman'd RAYBESTOS FRC10994N and RAYBESTOS FRC10993N for $60 each. It's the only place I've been able to find NEW not rebuilt. I've had issues with rebuilts in the past like leaky banjo bolt holes or leaky bleeders. Don't have to deal with core charge BS either.
I've done the rears on my 02 Blazer, 02 Crew Cab and 98 ZR2 extended cab in the last year. Learned from my mistakes to just do it all the first time or you'll be back in there again.
Had to go back into the Blazer when it was -10°F and I had the flu. Talk about pist.

 

Check the caliper 'pins' for sticking in the bracket. Check the caliper piston for sticking. I mentioned the caliper pins and brackets first because you can buy just the calipers, the calipers with new brackets, and even the complete calipers, brackets, and pads. I am not sure about the RockAuto.com deal.

 

If you do the job the cheapest way possible and it doesn't completely cure the issue, when you buy you second set of rotors you'll learn the same lesson I just did. On a 99 with 200k the calipers, pins, and pads are all way over due. Most calipers include the brackets and don't usually cost any more then just the calipers alone. BTW the original pads were ceramic.

 

I would check your 4wd.
Sounds like it could be partially engaging. Check your tranny fluid and transfer case levels.