Transmission Issue, need help

I just got a 99 Nissian Frontier a couple days ago. It shifts at high RPMS, I don't have a tachometer but I'm assuming somewhere between 5 and 7 grand. It doesn't slam into or jerk into the next gear. I'm not convinced it has a 3rd gear. Shifts into second around 22-28mph. I've got it up to 70 and haven't heard/felt it shift into 3rd. Read it could be a possible sensor or vacuum. Worse case scenario rebuild the tranny right? Just trying to start with the small things first.

Nissian
Frontier
1999
Automatic
Doesn't overheat
Idles smoothly

if its a overdrive transmission try manually downshifting into 3rd at about 50-60 mph and see if engine speeds up

I only have 1,2,D,N,R,P. First works, second doesn't shift at all front first, then drive shifts. So I'm guessing I have no second gear.