I bought a 95 blazer 4.3 fuel injected and when I bought it the man said it wouldn't catch third RU s fine through first and second but catches natural when it goes for third I had the transmission replaced but it still won't catch third please help any info would be appreciated!

 

Runs fine and catches neutral lol the guy who did the work said one transmission was electrical and one wasnt? I'm by no means a mechanic lol.