Did you swap in the gauges from the 91, or are you sure the reading on the tach is correct?
You might also check the throttle cable itself, and the throttle body. If the cable isn't tensioned correctly, even with the pedal fully depressed, "WOT" isn't available since the throttle body isn't ever truly wide open. Hence the "zip tie mod" on Explorers for a stretched gas pedal cable.
The rev limiter on the 4.0 OHV is set just below redline, which shows as ~5400 on some tachs, but is usually just shy of 5000 on the 91-94. The engine will rev freely up to the limiter in neutral, but hitting it in gear is still pointless for performance in terms of acceleration. Letting the transmission shift up around ~3000 RPM and going for quicker shifts from 1600-3000 RPM is a better way to get up to speed rather than some monster rev job until it hits the limiter every time. I know some people drive like this, but it just makes noise.
Check the fluid level in the tranny. Check out the fluid and be sure it's not black/brown or burnt. You might also want to be sure the engine is in top shape, with maintenance items like cleaning the MAF sensor, throttle body, and making sure the spark plugs are in top shape.
Bigger MAF's don't always help. Without internal work or better flowing heads (stock heads on 91-92's are weak and prone to cracks anyway), and a cam to go with it, a larger MAF can just screw with the ECU and actually give worse overall performance.
Aftermarket heads, a cam (maybe), and headers will go a long way to improving overall mid-top end performance if that's where you want the torque and power. The stock 4.0 has it's torque down very low.
A CAI like a KKM, or even a cob-jobbed adapted K&N cone filter will make a big difference. Sounds badass when revving, too.