I am a noob, so now that is out of the way, here is my post.

I have a 1999 Trek 7500 hybrid. When "getting on the pedals hard" it feels as though it is slipping a lil bit. From watching YouTube and looking on the internet, would I be right to assume that the pawls inside the freehub? are broken or gummed up really bad. I have never had one of these apart so it would be a learning experience. If parts are involved, would I be just as far ahead to get another rear wheel assembly?

What noise, if any, happened with the slipping?

We need to better understand the "slipping" you are feeling to figure out where the problem is.

does it only do it in a certian gear? If you free wheel the bike while the tire is in the air does it freewheel? It could be anything from chain to cassette wear. Its hard to know without seeing it may be worth takeing it to the lbs if you cant figure it out. Or if you had cone wrenches and aprobaly a 10mm allen key you could check freehub

Can you hear the pawls click when coasting?

It was only doing it in the smaller cogs, when more pressure was applied . I am planning on getting a new chain for it, I already have a new chainring and I have since taken the cassette of and converted to single speed with the use of a tensioner and it is still happening.

Could be the old chain is very worn so you are feeling chain skip. Since you plan to get a new chain anyway, see if that solves the problem.

New chain is on and it is still happening. I can hear the pawls clicking when the bike is upside down and turning the rear wheel. I haven't torn it down yet but is it possible that the ratcheting system in the freewheel has a bad spot and only when it contacts that one spot it acts up? Can you just get that part or have to get a whole new wheel?

It could be a s simple as the pawls needing lubrication. I don't know your hybrid, but I will describe what we did for decades to keep freewheels running. Perhaps some posters here and add to or correct what I have to say.

We used to take to wheel off the bike and lay it down drive side up. We would then take a can of light oil in one hand and spin the freewheel with the other, drizzling oil onto the crack between the FW body and the spinning FW until we saw that oil was coming out between the FW and the hub at the spokes. If what you have is gunked up pawls, this might well solve the problem. My experience is that doing this periodically keeps FWs (and so far, my cassettes) behaving nicely for years.

Ben

On a cassette hub, I'd be concerned about the oil diluting the drive-side wheel bearing's grease, and washing said grease into the freehub mechanism, gunking it up. It's not hard to remove Shimano freehubs and lube them individually.

If the condition is only on the smallest cogs, it can't be the ratchet pawls. When you shift to larger cogs and accelerate hard you exert much more force and slippage would be more likely

You need to replace the chain AND the cassette. Only replacing the chain means that a new chain is running on a worn cassette, which will make the problem even worse than before.

If it's only happening on the smaller cogs it can't possibly be the freewheeling mechanism. The smaller cogs on the cassette will wear faster, causing this issue.

As of now, it has a new chain, chainring and rear sprocket. The problem started when it had the old components on it so I decided to mess around and convert it to single speed. It is doing the same thing now as before so i would think that the only thing unchanged is the culprit . Earlier today I sprayed some PB Blaster in the area to hope it maybe breaks down the gunk in case the grease turned into sludge and isn't allowing the pawls to grab properly.