Harbor Freight has the rejuvinator's sometimes, I could not find them on the website though.
The short version is that it is a high pressure hose and coupler that you latch to the grease fitting, and you strike the back with a hammer which forces the oil, diesel, weasel pee of your choice through the hose, through the zerk and hopefully through the clog.
For me, I don't think mine has ever worked, although maybe it helped.
I am in the camp of first I try and get all load off the thing I am trying to grease, use jacks, stands, etc. etc. so that the joint is free, then I try moving the joint as I try and apply grease, being careful not to get pinched is important.
Then I remove the fitting and put the fitting on the grease gun and give it a squirt, usually it squirts through cleaning the zerk, if not, just change it as they are cheap. Then I use a pick, compressed air, cleaners and various weasel pee to break out the gunk that was under the zerk and usually leave the hole full of weasel pee of the day, I like Aero Kroil but PB blaster, yeild whatever your little heart thinks is the absoulute only one that works is fine, then I put the fitting back in again with the joint unloaded and try and grease again while pivoting / nutating the joint. (yes, that is a word)
If it will not start passing grease through then I take the joint apart, or just get so sick of fooling with it that I hope the mechanic fairies come in the middle of the night and just heal the thing...............
I seldom use heat as I seldom want a torch around the joints I am having problems with, usually rubber, grease and POL products are involved on a piece of equipment I cannot afford to replace so as a rule I tend not to apply a red wrench unless I am convinced the mechanic fairies are not flying and am desperate to have the thing work.
Good luck, around my place these usually involve a liberal dose of less then socially acceptable verbs and nouns and various denegrations of the parenthood of the design engineer for that piece of equipment when all too often to find the fault I must first find a mirror.