So, my 87 centurion (shimano 105) had been giving me alot of chain slack when coasting, the cranks were turning when walking the bike forward and it just felt sticky if that makes sense. So i sprayed some WD-40 in and on the rear cogs and in the back by the spokes where it connects. So now all those problems are gone and it is working good but it feels dry and i am wondering how to get grease in there with out removing it

You need to flush the Freewheel (FW) with light oil. WD-40 is a solvant and took care of the problem of gummed out FW.
You need to add some lubricant using the same technique.

would a good quality motor oil do the trick?, and is it even possible to get grease back into it with out removing it entirely?

Well...the short answer is, you can't. What's happening is the grease that is keeping the pawls inside the freewheel body moving freely is pretty much gone, and what is left is most likely very dry and full of years of grime and grit. You can remove the freewheel, flush it out and re-lube it with some heavy lube. My favorite is Phil's Tenacious oil. The reason why it feels "dry" is that really, it is. WD-40 is NOT a lubricant...it is a water displacement and penetrating fluid. Basically, you've actually removed the lubricant with the WD-40. The other thing you can do is simply buy a new freewheel and chain and call it good. '87 is six speed, yes? You would probably have to look around a bit, but they are out there, just make sure it's a compatible "twist-tooth" freewheel. Technically, you can actually take the entire freewheel apart and lubricate it "properly", however that is a very cumbersome job. I was never a fan of it, even as a professional mechanic. Whatever you choose, good luck!

ok so suppose i had a shop take it part, what would it cost me?

and i know wd40 is not a lube and that it would strip the grease, it was getting really sticky and i had no other options really

Completely depends on the shop. But, IMHO....too much.

yea thats what i was thinking, is it that much a pain to take off and grease myself? im in summer vac. so i have alot of time haha

wait im kind of confused here, is a 1987 shimano 105 a free wheeel or what else is it?

Actually, that's a good question! It may indeed by a cassette. If there is a "bulge" in the hub body, it's a cassette. Also, if it's an actual 105 hub, that probably makes it a cassette as well. In that case, you would just need to replace the freehub body, which is a very easy job...although it does involve removing the axle and repacking the hub bearings. And, you'll need a 10mm allen with a rather long extension...and you'll need a new freehub body.

1. Take your back wheel off and set it on top of a trash can with the freewheel pointing up.
2. Dribble some light oil in the freewheel crack between the part that moves with the cogs and the part that's fixed to the hub.
3. Give it some time to soak in.

That's kind of a quick, dirty solution to your problem but how much are you going to spend on a 25 year old bike?

well im really kind of attatched to it now haha, i like the vintage rides over the new ones for some reason lol, but what exacly do you guys mean by a bulge, and "light oil", are we talking 3 in 1, motor, or what(i have good motor oil)

and how much would a free hub body cost just estimate?

the thing is im pretty sure the grease in there had disintegrated and or dried up completely or at least nearly which was the cause of the sticky/gummed up feel i was feeling, now that the dirt and messed up old grease is gone i can backpedal fine, coast fine, (which i was previously unable to do) so im thinking tomorrow ill flush all the old crap out with more wd40(this pretty much is the only viable option, the machanics at the shop said it would not be their time or my money to mess with it, the most they said they could do was squirt some oil in there, which i had tried already to no avail. ) so after its all flushed out ill let some motor oil or something of its ilk soak in, and i mean alot of it, and then just prey i can get a little bit of grease to get taken in by the oil, sound like a decen enough plan?

Any high detergant oil will do. I use air tool oil. It wicks in very tight spaces. I remove the FW. Put is upside down on a dirty rag, and drip oil from the back side.
The oil and dirt will flush out from the front.
This way you are flushing dirt out.
You can also drip it from the front if you don't have the tool to remove it which was also mentioned.
Now that it is working, I wouldn't worry about it too much. When you are pedal backwards there is not much stress on the Freewheel.
If you do own a vise, I would recommend you buy the FW removal tool. Sooner or later you would need it to clean or replace the FW.
Enjoy the Centurion Ironman. They are great bikes and we have a few experts here on them.

This is a freewheel right? The cassette screws onto the hub(?)

If so, you can, like others mentioned, drip some lube down into the crack between the lockring and the freewheel body OR you can disassemble it. Sheldon Brown has a procedure to do this - here.

The "drip in the lube" method is what I had done in the past to my old winter bike. But I had taken the freewheel apart to see the guts of 'er; cleaned it up, regreased and remounted and it worked fine until I replaced the wheels with a freehub.

It is a good way to see the innards of the freewheel and an educational component as well. However, it would be less trouble to just buy a new freewheel.

You really had a couple of good options: remove it, flush it well and lubricate it; take it to a shop and have them do the same; or replace it. Six or seven speed freewheels are available cheap.

I would buy a couple of tools, and do it yourself. Cleaning and lubricating a freewheel is one of those basic maintenance items that most people can do with minimal practice.