1994 S10 SS trans problem

I have a 1994 S10 SS with about 74,000 miles. It's a 4.3 vortec v6 with the 4L60E transmission. It hasn't been shifting right and after about 55 mph when the accelerator is pushed to the floor it revs up. I though it might have had water in it because the radiator always has to be checked for coolant and add coolant to it. But when i changed the transmission filter and oil I put a trans cooler on it and blocked off the radiator. But the oil just looked dark red and had a little debris in it. So I think something else is wrong and I am hoping to God I don't have to rebuild the transmission. But I am wondering why would it act like this with only 74,000 miles on the truck and transmission. It was my grandpas before I got it this spring and grandpa isn't a burnout junkie. I am 17 and don't make alot of money.

some trans component is slipping as indicated by the revving up of the engine, debris and dark fluid. the tcm will up line pressure to protect the trans and this causes harsh shifts. check for dtc and post them.

I kinda figured it was slipping but it only slips at high speeds. I was trying to figure it out earlier it doesn't rev or want to slip at lower speeds. And I don't have a OBD scanner for diagnostic codes.

does it only slip in 1 particular gear and is normal in all others?

Well I seems only to slip when it down shifts to 3rd gear.
It does seem like it doesn't slip in the rest of the gears but it shifts on average at over 3,000 rpm and it shifts better when you let up on the accelerator a little bit.

Only slips in overdrive tried it in drive and doesn't slip.