2003 Bravada Front Diff Replacement Gone Wrong, HELP!

The front differential on my 2003 Bravada went out. I had brought to local mechanic who ive went to before. After 3 weeks or so I got a phone call yesterday. The mechanic said that its finished. I went and paid for it, $1200 for diff and labor. Pulling out of mechanics parking lot about 1/4 mile down road, oil pressure light came on. Light states to shut off engine immediately. I did that, checked oil dip stick, full of oil. Before starting back up I called the mechanic. He said that the oil pressure light was because the dash gauges are all messed up. They have been bad since I bought the bravada 2 years ago. Even they are bad I never had warning lights show up. He swore thats the issue and nothing to be concerned about. I was worried since like I said, I never had light come on for anything. I took his word for it and drove truck home. 1 hour later I left home for work and made it about 2 miles or so and truck running very bad. Knew right away, Oh Crap!! ITs the low oil pressure and he was wrong. Turned vehicle around to go back home. Didnt make it 50 more feet, truck shut off. The motor never turned over again.

Now I'm not a mechanic but did some digging and saw that he would of needed to pull off oil pan for the front diff job. So, can use some feedback! What did he do wrong? Am I right to think that he forgot something on accident? If yes, would he be liable for something. Like I previously wrote, I just paid him in full, $1200 in cash. 2 hours later and no more truck again. :sadcry:
FEEDBACK is much appreciated please, please. After truck broke down, I called him and he towed it back to his garage. Is motor junk now?

Thanks so much guys, and gals
Aaron Lesperance
2003 Bravada AWD, Front Diff Replacement