2000 trans slipping and speedo gone haywire

I have a 2000 s10 with 135k 4x4 6cyl extended cap. Truck was sitting for about a year. Decided to get her going again. New tires front end work and a few emissions things to pass emissions. Truck runs pretty decent. This morning the trans started skipping. Check fluid and it was low a quart. Haven't noticed it leaking but it's possible. Truck is better after the fluid but after a longer test ride it's skipping still. Had to manual shift it a few times to get it home. Not sure if it's related but speedometer is jumping all over the place when it's skipping. Is there a modulator valve or something it could be or is my transmission toast? Anything to check before it goes to a shop or for sale as is?