Hi,
I agree, just try using the grease gun first.
However, those bushings are tight fitting and often hard to get grease into. That's why some folks use 90W gear oil (injected with a special gun such as the Mercury outboard motor oil gun, or a something homemade) instead of grease.
The factory manual does indicate these joints should be greased, but I believe there was a later bulletin about using 90W gear oil in them instead, if grease can't be injected.
So, go ahead and try grease, first.
If the grease goes in really easily, that's a clue to check your trunnions for excessive wear and play. Shake the tire side to side and see if you can feel any play. Or, better yet, have someone else do the shaking while you watch the joint with a flashlight. There are other joints in the front suspension that can lead to play, it helps to narrow down which one is loose by watching the suspension.
If grease doesn't go in easily, there are several things you can try:
Put the front of the car up on jack stands securely. Place a jack under the tire and have a helper move the suspension up and down a little while you keep pressure on the grease gun trying to inject the grease. Note: be careful not to jack the car enough to de-stabilize it on the jack stands. You're just trying to get the trunnion joint to move a little, up and down, while injecting grease. This often helps.
Sometimes an air-pressure driven grease gun will do the trick, if you have access to one, when a hand-operated gun won't work.
Warm the grease by leaving the grease gun out in the sun for a while.
If it's been a while since they were lubed, it might help to use oil first, then follow up with grease. Of course, that means getting hold of or making an oil gun.
BTW, I've heard that when replacing the trunnion bushings, if they are slathered up nicely with 90W gear oil before assembly, greasing will be much easier in the future.
Cheers!
Alan