I bought and have used a cheap and simple hypodermic greasegun attachment commonly available at NAPA to inject grease directly into the balljoints and other non-greaseable suspension parts thru the rubber boots:
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Many owners - me included - have found there is VERY little OEM grease installed into these joints direct from the factory, leading to premature failure. The needle route so far has been totally effective, at least on my truck with 50K miles, and a full suspension still tight as new.
Some alarmists get all excited and critical over the "danger" of piercing the sealed boots to apply grease by the above method - but make your choice, lovely and unspoiled pristine boots and
prematurely failed joints, or pierced boots and joints that will last
MANY tens of thousands of miles - besides - don't forget that the greasable joints for many decades were UNsealed, and seemed to last quite well with decent service!
Here's a shot of the needles I refer to:
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Ya pays yer $$$ in life, and make yer choices!

:-laf