Hello dseretkis,
Heat is a good direction to take. Be careful of the rubber seals. You can also unsrcew the grease nipples and clean out the cavity, spray some penetrant into the cavity and re-assemble. Test the grease nipple with the grease gun before re-installing it. Attempt to grease the joint after the nipple is back in place. If it does not take grease drive the car a bit to allow the penetrant to work then try to grease.
You can buy a grease injector which you impact with a hammer to force grease into clogged grease points. Look on line, also Harbor Freight sells them. I think they are less than $20.00
Keep working on it.