Transmission gave me a P0777 code?

So I bought a Suburban from a hole in the wall dealership about 6 days ago. Before I bought it I hooked my scanner up to it and it gave me a P0777 code. I questioned the dealer about it and he said the transmission was rebuilt and he did not have the paperwork but he gave me the number to the shop the did the rebuild and said I can call them. He gave me a 3 month 90 day warranty on the engine and trans and said if I have any problems I can bring it back to him or any shop. When I drove the truck off the lot I started to smell something burning. I figured it was just from the truck sitting for a few months. Drove it home, no real issues. A few days later, the truck shifts weird. Not all the time but it feels like the shifts aren’t consistent, some are shorter, some are longer. It rumbles slightly between 45-60 mph. Not violently, just a slight rumble. My wife drove it today and when she picked me up it hard shifted twice on the 15 min ride home. Never did that before, it shifts smooth 90% of the time. The trans temps haven’t gotten over 169. I have to take an emergency trip back home and will be driving 800 miles so I figured I’d check the transmission fluid, it was bone dry. What should I do now