I'd look at job postings to see what requirements they list and then research how to obtain those. I'd call the places you're most interested in and ask to speak to the shop foreman. Ask them what training they value and from which school.
There are trade school and maybe apprenticeship programs where you can get certifications. This route may have a significant costs, but may increase your chances to get hired and starting a higher salary. You'd need to do extensive research on 'for profit' schools to ensure employers really value the certification.
Community colleges and Universities may also offer a more generic Auto Mechanic program and then you could specialized in Diesel. https://www.universities.com/find/texas/best/diesel-mechanics-technology-technician-degrees has a list that includes Western Technical College in El Paso. However, they didn't make this https://www.niche.com/colleges/search/best-colleges-with-diesel-mechanics/ list.