2001 Chevrolet S-10 transmission

I own a 2001 chevy S10 4 cly, pickup. I recently took the truck to the chevy dealer and complained about how the car was running. I asked them to go through it and fix anything that was wrong, just contact me first. The list of repairs included, Replace the camshaft gear, the distribution chain (timing Chain), chain and sproket, replace rear crankshaft seal, also the rear crankcase seal, chain the oil and two tubes of silicone? When I got the truck back, it was stuck in first gear as I drove away, I went around the block a large block and straight back to the dealer. I was told that they forgot to plug in the transmision shift plug? However, I also saw 2 mechanics placing a liter of transmission fluid in the truck. When I mentioned it, they said just topping it off not a liter? In any case now the truck has slipage or a fell of not holding 3 or 4 gear, at 50 to 70 miles per hour it sounds like it whines or slips back and forth. Any ideas if the previous work would have in anyway damaged my trans or involved my trans? What about being stuck in 1st gear? Why the trans fluid? Or is this something new and just what happens. I am going back today and really know nothing about transmissions. Thanks ahead of time