You have to keep in mind that it's a 4 cylinder truck, not the biggest on power to begin with. I have a 1998 ext. cab 2.5 and it will slow down when going up some hills. Pretty normal. Downshit into 4th. After I did all the mods I have done, it made a HUGE difference. Now depending on the size hill I am going up, or the load I am carrying, I still have to shift down to 4th every now and then.
Have you done anything to the truck or it is bone stock? How many miles are on it? Recent tune up? Also keep in mind you said you are doing about 60mph in 5th gear, that is great if you are flat, but up hill it's just going to chug. I normally cruise on the freeway at 75-80mph.
1998 Mazda B2500 Extra Cab 5sp manual
(1998 Ford Ranger 2.5 liter)
Cat back with Flowmaster
Headers (no name)
Power Aid TB spacer
JET Performance chip
Electric Fan coming soon!