Question Replacing my fifth gear?

[gsedun]

I have a manual 99 4runner SR5 v6, and when i drive at 120kmh(74.5mph) i am at 3000rpm. The 4runner does not seem comfortable at that speed..as soon as I take my foot off the gas it wants to go down to 100kmh(62mph). I can cope with the first 4 gears..my question is whether it is feasible to alter just the fifth gear? I dont want to alter my other gears or gear ratio because that would mess up the transfer case ratios etc etc..I do mostly highway driving close to 60km(37 miles) a day, but i also go play on dirt/mud/snow roads..