Why won't my spark plugs tighen or loosen?

Car: Ford F150 xlt 1991 100xxx Engine: "crate" v8 about 50xxx miles, (looks like original engine)
The car was just bought so I wanted to look at the spark plugs to see if they can tell me anything about how the engine was running. I removed the spark plug cable, then with my standard spark plug socket I applied it to the spark plug and went to loosen it. To my surprise it was already loose, after 30 full turn of the socket there was no difference made… The spark plugs was still on the head.
Its like I was tightening a bolt into rubber… No difference could be made.
So after giving up I went to tighten the spark plugs and it would not tighten. So I put the plug wire back on, now after 25mph the engine shakes a little.
What's wrong with the spark plug?

Some of the Ford engines had a two piece plug that had a habit of breaking when removal was attempted. You need to take it into a Ford garage where they have the proper tools to remove and replace the plug. Unfortunately it may be a bit expensive. FORD's "Better Idea".

Now you're getting the idea why Ford discontinued this engine.

Sure sounds like stripped threads to me. The only fix is to drill out the hole and install a "Timsert" insert. You Can't use a helicoil on a spark plug hole.

Wondering about your "standard spark plug socket"? What size is it? 13/16 used to be standard, but Ford went to a 5/8. I think in '80's, on some of it's engines. Now days there are many different sized plugs depending on vehicle and if metric or not.