94 k1500 transmission problem?
 

So this truck is new to me.
I aquired from my dad who got it from someone else.
It sat in my dads yard for couple of years and sat before that for quite a few years at the owners before him.
When my dad had it he said it started up, ran and drove before he let it sit. He said that it got really bad mileage and it seemed to not want to shift out of second.
I got it, it wouldn't start and ended up having to replace the distributor, battery and ground cable off the battery.

It also has an engine from a 77-79.
So it's missing the knock sensor and has some other error codes I need to dig into.

The most pressing issue now is that it feels like the transmission is slipping.
I'm not really sure how to explain it as I've never run into this issue before.
I step on the gas in drive.
The revs go up but the truck barely moves.
Same with going up a hill.
Revs go up but the truck has trouble getting up the hill.
When I am driving it on the road it will eventually get up to speed on the flats and feels like it shifts fine and drives smooth.
It just has trouble when I'm getting going or going up a hill.
Almost like a clutch is half engaged.
Same with reverse.
The fluid is a like a plumb color. Very dark purple.
I have changed out the filter but it hasn't helped.

I'm going to try doing a complete fluid change.

If that doesn't work then the torque converter.

Any thoughts?

Thanks