I have an 03 blazer 4 door 4.3L LS with 4wd. I just started having an issue accelerating especially up hills. My rpms go up but it doesn't shift gears probably just due to the slow speed. If I'm trying to go uphill I almost won't make it and I'll have to crawl up the hill going 25 or 30 mph. If I try to floor it the rpms will go up but itll decelerate even faster. Anybody run into this before or have any thoughts on why this started happening?

 

When my 04 was doing something very similar it ended up being a clogged cat converter. Never did throw a check engine light or anything, just noticed one day when I tried to pass someone on a two lane road, I laid into the throttle and got over to go around them and the truck actually slowed down...

 

This would be my first suspicion.