Inner CV joints, specifically the right side. That is the one that gets "cooked" the most near the exhaust. There is a heat shield there that helps prolong this, but eventually they need to be service.
What I like to do is first see if you can lessen it by getting some grease in the joint. I have a special tool just for this purpose:
It allows me to slip some grease in past the boot via a grease gun. Often this is all they need, although some times it takes some driving to get the grease "worked in" to the joint, and help soften the grease already in there that has turned into a thick black wax.
But in some cases, the joint just needs to come apart, get cleaned, repacked with fresh grease, and reinstalled.
If I go through the trouble of the latter, I go ahead and do BOTH sides, and swap the joints left to right in the process, so you get a new bearing surface on the loaded side of the joint.
DO NOT INSTALL AFTERMARKET DRIVE AXLES