i noticed that my u-joints are held in place with this odd plastic stuff. i know i have to heat it up to burn this out, then remove the joint. but my question is...how do i reinstall the new one?

well an aftermarket u-joint just come with clips or something i can use instead?

 

The molded plastic retainer is factory installed. Using a simple propane torch is all that's required to melt it out so you can remove the u-joint.

The new replacement u-joint will come with metal retainer clips to hold it in. There's no way to reseal it with plastic.