It's not broken, that's just how it works. The torque converter is a fluid connection between the engine and the transmission, replacing the clutch in a manual transmission. The slower it goes, the more it slips. It's want lets the engine run while the truck is stopped in gear. A lockup converter has a small clutch in it to eliminate the slip at speed, which is roughly 300rpms at 3krpms engine speed. It's for economy, to eliminate the mpg lost to tranny slippage. The clutch locks up at about 42-45mph. In some trannies, it will unlock if you step down on the gas (a GM 700R4, for example), like a semi passing gear downshift. In the montero, I don't believe it does that, but if you drop below the lockup speed, it will unlock and the engine rpms will rise by 300-500rpms in the speed range you are quoting. It is also possible the tranny is downshifting, as the o/d gear is quite high for that speed, and the tranny really wants to go to 3rd. Manually downshift to 3rd at that speed and report rpms...