82 olds 98 possible trans issue help?

ive got an 82 olds 98.. This weekend my speedometer quit working and my check engine like when on.. now onto the part that worries me the most.. Before this weekend normally my car would at around 40-45mph shift up into a fourth gear and i could hear the cars rpms drop really low and it would just cruise almost like an overdrive gas saver.. but now the same time that the speedometer went out the car hasnt shifted up into that fourth gear since saturday morning on my way to work.. I dont want to have to take it in but im probably going to have to.. being only 17 and paying for everything with basically no help from my parents at all im trying to save the money.. if anyone could help me out and knows what might be causing the car not to shift.. o and it not like the car slips.. it drives as if that gear never existed doesnt even try to shift up.. but i thank anyone who might know something that may help

Welcome to AF. No speedo, no 4th/od. Get that fixed first.

ok so speed censor right? is that an easy or hard fix and where exactly is it located?? thank you so much!

In the trans in the tail housing, check wiring to it first.