2004 Ford Explorer Sport Trac V6 Stick.
4th gear, 4000 rpm, 40mph and climbs in speed slooooooowly. It almost feels like the clutch isn't engaging all the way. There is a bit of a burning smell that almost smells like the belt, but the belt looks fine and as far as I can figure, isn't directly connected to the problem. The math doesn't add up between the rpm and the power output. I'll be changing the oil on it as well because I think it needs it anyway, but I'm not sure if old oil could cause this sort of an issue. Looking for some guidance before I start tearing things apart.

Most recent maintenance was replacing the Fuel pump in the tank. Was having trouble starting. This issue only started up about 3 days after having replaced the Pump but idk if they are connected or convenient. Get's plenty of gas, cranks fine, rev's fine, just doesn't transfer it to power output. I noticed this problem yesterday, on the way home it was significantly worse.

 

From what you describe , it sounds like the clutch in the vehicle is slipping.

All clutches should have some free ply varying from 1/2 in to 1and 1/2 in. That is the distance the clutch pedal goes down before you feel and resistance/pressure.
There is an adjustment for that , but if the clutch assy has been destroyed from slipping, replacement of the pressure plate, disc, and release bearing are required and this is not a simple job without the right equipment. Read up on how to adjust a clutch and you can try it to see if it is not too late.