Semper Fi brother!
Welding and diesel mechanic, do I know about those. Spent 18 years cranking wrenches. Rebuilt anything that could be rebuilt on a Cat. Engines, transmissions, rear differentials, front differentials, hydrostats, hydraulic pumps, motors, cylinders and even rebuilt a Detroit Series 60 at the Cat dealer. They asked if I could do it and I said, yep. Rigged up a wiring harness, controls to run on the dyno. Worked at Western Star dealer, Case heavy equipment, Dodge dealer, nationwide equipment rental place doing field service and finished off at a Cadillac dealer before waking up and realizing I was tired of being a mechanic/technician.
Welding as a mechanic. In my experience in the dealerships(cars/light trucks) everybody was a welder, using that term loosely. We welded on some exhaust pipe when a catalytic converter needed to be replaced and that was about it. The body shop on the other hand welded more.
Welding as a heavy equipment mechanic. At the Cat dealer they had their own welding/machine shop so unless you work in there you wouldn't be welding. Welders there were welders with some mechanical but usually broke stuff was removed and taken to them. They did have a field service set up for a welder to go out and the machinists could weld and would get sent out at times to repair pivot points on machines.
In the back of the shop were the grunts. They disassembled components, repaired tracks, did heavy duty work. Did some minor stuff on engines, checking pressures, diagnostics and such. In the front shop we called it the "Spec Shop" we rebuilt components. Had a hydraulic test bench for testing hydrostatic transmissions, pumps and motors after being rebuilt or for diagnostics. We also had an engine dyno for testing rebuilt diesel engines. The only reason we welded was to fabricate something for home after work in the welding shop. We made some crazy special tools for picking up valve bodies from transmissions or other things.
The Case dealer had their own welding shop, again, term used loosely. It was one bay with a grump old welder who stood around talking more than working. After he quit they moved an in house guy into that position, me, then sent me to school. I was the young grumpy welder who later told them to stick their job up their backside...thanks for the education though.
Welding in Truck shops dealers, similar to car dealers, everybody was a welder unless it was something serious like fixing an axle, frame repair where us unqualified wannabe's were not allowed. Nothing real serious got done in my experience and if it did they hired out for it.
Mechanic in general. I did if for quite awhile but think I got burned out. Worked flat rate for nearly 5 years and got tired of Union wankers designing, making steaming piles of crap then bending me over on warranty times that some other union wanker thought up by picking a number out of a hat. Worked my tail off, was a GM Master Technician, electrical, engines and more, ASE Certified Master Tech and ASE Diesel Tech. I took my automatic transmission test. Knew I should study but didn't. Sat in the car the night of the test looking over the book for 15 minutes. Went in and took the test. A few weeks later found out I passed. I hadn't touched an automatic transmission in over 10 years. LOL!!
The rental equipment place, all I can say is if you become a mechanic, run like hel from those places. If you think, "maybe I'll check on a job there", my advise, keep driving.
Overall I did enjoy it, after that much time there is nothing I'm worried about taking apart because I've likely had more complicated apart. I bring these things up to show that I left on my own, not because I couldn't "hack" it. It's fun to work on things now and again, rebuilding old tractor engines, continental engines, play around with some cars.
Tools- Oh lord, you become a mechanic, you'll easily tie up $50k in tools. When I left I had around $60-70 in tools and tool box. Drove a 90's model Saturn....used.
So, welding in the mechanic field, not a real necessity in my experience and if you put it on a resume when you apply at the Kenworth dealer they really won't care. Oh, truck dealers are usually flat rate. If you are not familiar with flat rate that means if there is nothing to do in the shop, you are NOT getting paid. I repeat, not getting paid. I remember clearing off the top of my tool box, taking my coat and folding it into a pillow and literally going to sleep. Some days I would just leave. They would ask me the next day, "where did you go??". Home I would say. "Why, you were supposed to be at work". To which I would say, "why, so I could sleep on my toolbox doing nothing".
The only aspect of welding/mechanic would be a field mechanic. Used to make things, do minor repairs and such but I don't think they are responsible for any serious weld repairs. They are mechanics that can lay down a pretty good bead usually. Lots of dirt, lots of grease and lovely places like landfills to repair tracks on a D8, watch out for needles and hep 3.
I checked on that Helmets to Hardhats and pretty good deal. I went thru them to apply for the boilermakers union, got put on the list for an apprenticeship at #2. They called about 8-9 months later and asked if I was ready to go to work. My business was booming and turned them down.
Out of all of the Unions, if you want to go that route, go Boilermakers. Best pay starting out. I would have started around $18/hour and full education. Everybody else, Pipefitters, Ironworkers, starting pay sucked. Top pay for a Journeymen Boilermaker also blew away the others.
Good luck to you, any questions, shoot me a message here.
Shawn
However, I'm really considering enrolling in a one year welding cert at a local community college in order to give the trade a try. 
