93 Sierra no first gear/kick down

[Pjalls89]

Hey guys, new here, would love some help, I’m at a loss. I was a GM tech for 8 years but I didn’t delve into “older” vehicles that much. I recently bought a 93 Sierra 1500, 5.7L, 4L60e, 4x4. Truck starts up great for sitting for 2 years. He parked it because transmission “wouldn’t go” I drove it home a mile down the road. Truck idles smooth and clean, runs good. My issue is, it won’t go into first gear normally on its own or by manually selecting first gear. It also won’t kick down when driving, it always starts off in second, runs up through the gears and when you slow down it will drop back down, but if you floor it to get it to kick down, like your gonna pass, it doesn’t do anything.  You come to a stop and it feels like it starts off in 2nd gear. I have changed both square solenoids, changed the downshift solenoid, checked the accumulator spring and piston in the transmission and now I’ve even replaced the computer. It had a service engine light on, I flashed out the codes with the ol paper clip jumper, had code for knock sensor and throttle position, found knock sensor just unplugged, replaced throttle position sensor, unhooked battery and ECM and no light has come back on. Most recently when I changed the computer out, I had first gear when I pulled out of the driveway, she chugged twice before going into second and then after that she sputtered and chugged and it didn’t feel like she shifted out of second or 3rd maybe on her first test drive. I don’t know if I have a transmission related issue at this point, power issue from the engine, or maybe something else that I just am not aware of on these older trucks. I’ve heard change the speed sensor, could be vacuum, it’s in the valve body etc. I just don’t know, but I don’t want to just keep loading the parts cannon and shotgunning stuff at it. But I don’t know how to diagnose it because it’s OBD1 and I’m a spoiled brat with tech2 and gds2 computer diagnostics Thanks for any help!