GMC Sierra transmission

[Hunter nh]

Hello, I'm at a complete loss and need help, I recently bought a truck and nothing but problems. It would reach 20mph and the speedo would drop to 0 both abs and parking brake light come on. The truck gets stuck in 4th gear and loses power climbing hills. I changed the vss no change then I found a pinkish wire under the hood that was loose and push it back in with electrical tape it runs a bit better but not fixed.
I can restart the truck and it's all good again till I get up to speed. I've stopped already 30+ times just to reset the truck.
Also the fuel pump started leaking out of the top while filling up. The guy I bought said he replaced it I checked it all wires seems fine but the main white part is sorta loose? I'm not really good with trucks but I'm trying to get across country rn and all the mechanics near me are booked till the 25th any advice thanks.