One problem after another. TH400 shift issues

[Orion88]

Ever since I swapped out my original carb/intake on my 86 K10 Silverado I've had nothing but headaches. Took me forever to get the idle settings right, my automatic window got stuck in the down position, and I never could get the electric choke to work for a damn, so last weekend I pulled the 1407 Edelbrock and replaced it with a 1405. I knew the 1407 was too big to begin with but I had it sitting around and I wanted to learn how to rebuild a carb. Since it was too much carb I eventually swapped it out for the more appropriate 1405. Drove it from Indy to Bloomington the other night (45 miles, mostly highway cruise control set at 60mph), she drove smooth as silk and got way better fuel mileage. I had to kick it into 4wd to get into my snowy gravel driveway. I finally felt like everything was working right.

So I fire it up the next morning, pull out of my drive, turn 4wd off, and start driving to school. It doesn't take me long to realize something isn't right. The tranny doesn't want to shift out of 1st, and when it does it's at 35mph and it shifts HARD. I had this issue once and it was as simple as hooking up a loose vacuum line. I check all of the lines and everything seems fine. So I go to Advance and buy 12 feet of vacuum line, vacuum connectors and T's, vacuum plugs, etc. I replaced the line going from my manifold to the transmission vacuum modulator, and the line going from the manifold vacuum port on the carb to the distributor, and I replaced the plug on the timed vacuum port on the other side of the carb. That didn't do anything, so I replaced my transmission vacuum modulator (covered myself in tranny fluid in the process in the Advance parking lot). That didn't do anything either, so I spliced the modulator vacuum line into the line going from the manifold vacuum on the carb to the distributor. And still, the problem is not fixed. The only vacuum lines I haven't touched on the carb or manifold is the one going to the cruise control, and the PCV valve.

My guess is there has to be a vacuum leak somewhere affecting the whole system. All of the smog equipment is removed. I'm not sure where else to look. This problem literally popped up overnight and for the life of me I can't figure it out. Is my transmission ruined? I feel like it has to be something simple...