When you get to a point where it should make the 2 > 3 shift... does the RPM just climb / 'flare', at that point? Does it do it even when you shift manually, or put the trans in '3'?
If so, there's a good chance you have a bad 3rd clutch -- it's a fairly common thing with the 4L60E, especially if the trans fluid overheats. To fix it requires remove / rebuild (or replace). Plenty of threads here on trans issues, I'm afraid.
GM is known for having created some great transmissions in the past -- the 4L60E was put in just about every rear-wheel drive vehicle between (approximately) 1993-2013. But it's not 'bulletproof', and there are known issues with it. Some can be improved on. A couple can't be, due to design / engineering limitations.
That being said... if you keep it serviced at appropriate intervals (fluid / filter changes, using the correct fluid), it'll last close to 200,000 miles. Longer, if a few dollars are spent to rectify some of the more common issues (aka: corrective shift kit), which can be done with the trans in-vehicle (and many here have done it).
For now, though... you'll probably need to decide whether you want to cut your losses on the $800 you spent so far... or if you want to spend $500 - $2000 for another (junkyard, all the way to a rebuilt / remanufactured). It's not something that can be done by a novice, either.
Given the book value on a typical '04, fixing & flipping won't return the investment you make, either. If you do repair it, it should be with the idea that you're going to hold onto the truck and get the value of the money you spent to fix it.