I changed my rear end in my 84 c10 2wd from a 10bolt to 12 bolt but now drive shaft wont bolt up u joint is different size, do i need change drive shaft or is ther a u joint that will fit rear i put in is out of 81 c10.

As long as you dont have a length issue, u-joints are kind of standardized, in series-1310,1320,1330,1340,1350, ect & adapter joints that have two different cap sizes.
I'd remove the joint from the shaft & you'll have to measure the pinion flange or remove it & take it with you. Most citys have a driveshaft specialty shop & it's no biggy for them to set you up.
Or just go buy the measurements, cap size & width for both ends, I think they go up in 1/16" increments. No use going by application.