Lost second gear

Hey everyone. I have a 1989 GMC Sierra k1500 with a 350t transmisson. I bought a transmission brand new about a year and half ago. I had a 1 year limited 100,000 mile warranty. I lost 2nd gear at about the years time which made it go from first to third, unfortunaly my odometer doesn't work and hasn't sense i bought it so they threw the warranty out and wouldn't fix it. I usually fix my own problems but i'd like to know whats wrong with it, i know its not the kickdown cable or the vacuum? what could it be

Internal has to come out, either way.