My question is what would cause the plastic to melt? There is no heat source on a drive line. I suggest that the u-joint is bad (no grease)and heated up in the drive causing melting of the plastic.
Your theory sounds plausible. Is there an exhaust pipe or catalytic converter close to the U-joint? Is there extra play in the U-joint or is it making noise or vibration? Were you able to add grease? I suppose another possibility is that if the U-joint was replaced, that some truly clueless person applied heat to it for some reason to get it installed instead of pressing it in.
You would be correct. It’s a dry joint that is overheating.
R U referring to the rubber boot that protects C-V joints?? Is it a factory or rebuilt half-shaft?
Have you provided the manufacture of the part you question, at times this can be important. A simple,this is a GM u-joint,or this is a FORD u-joint would be sufficient. Just pretend you are at the local parts counter and give us the same info you would give them. Any chance of a picture, perhaps what you describe as melted is normal or left over from a previous joint change.