I use LMC truck parts all the time.
THE PROBLEM is they (and everybody ELSE) uses cheap chinese metal that in the rear tailgate fails almost EVERY TIME shortly after installation. It's not your body shop's fault IF that's what has happened. Giving them parts that fail upon installation is problematic at best.
Ask me WHY I know this.
Find some parts out of a civi blazer in a salvage yard. Clean them up (media blast or wire brush the rust off) and THEN install those. They will work properly.
And does the handle just spin when you turn it?
Chances are high you blew out the 'clutch' - see this page (46) in the LMC Truck catalog. Look at the top left corner and look at part #6
LMC Truck Parts - Page 46
THAT part is made of pot metal and will NEVER hold up to any kind of stress.
But without that part, your window will never go up or down normally.
Good luck, there are no places I'm aware of that makes that part in real machined steel. Just cheap chinese pot metal...