91 toyota 4x4 22re overheats on highway, but not at idle.

Sounds to me like stuck themostate replace then try it

sounds like an air pocket in your cooling system need to bleed system and add missing coolant

Have the FAN CLUTCH checked. they are a common failure. Get a AC DELCO or the part from the dealer. Why was the radiator replaced? Heater work OK? A/C works OK?

white smoke from the exhaust? does the oil look like choclate milk? sounds like blown head gasket. That is the only thing that will make a car overheat that fast

most probably the rad is getting plugged, or not transfering heat to the fins, get a new one, and replace the thermostat , fill with new coolant

Several things can contribute to overheating. If the vehicle doesn't overheat at idle but slowly begins to overheat at highway speeds then I would suspect a partially plugged radiator that just simply can't get rid of the extra heat generated at higher engine loads and speeds. The radiator could be plugged internally which would require it's removal and professional cleaning [or replacement] or it could be plugged externally with bugs, dirt, and other crud that is restricting the airflow through it which can usually be fixed by thoroughly cleaning both sides of the radiator with a garden hose. If, however, the engine runs at near normal temps at highway speeds but overheats at lower speeds then I would look at the radiator cooling fan operation. Once you are above about 35 mph there is sufficient air being rammed through the radiator to cool it but below this the cooling fan[s] need to work to limit how warm the coolant can get. A more rare cause could be a leaking head gasket which often will cause a random sudden overheat condition [often with a loss of heat from the heater during the event] that initially may come and go for several days or weeks before becoming severe. Make sure your cooling system is properly filled with a mix of 50/50 coolant and water and hasn't accidentally been filled with only 100% antifreeze and this too will cause odd heating problems.

here are a few things to check, coolant level low, drivebelt defective, radiator could be blocked, faulty thermostat, electric cooling fans not working which makes the engine overheat, when going a certain speed it is air cooled, but when you stop it will overheat.

Have you changed the water pump ?

Check for collapsing (soft) coolant hose on intake side of waterpump at engine above idle rpm.
Check condition of system pressure cap.
Check operation of cooling fan/s- fan temperature sensor switch on engine may be faulty.
Check system has correct fill of coolant and no air locks exist when re-filling- bleed off any airlocks as may be necessary.
Check radiator coolant core not blocked.

When driving down highway only, it overheats? When running steady, I take it you mean, sitting idling? If it's only overheating at highway speeds, and all else has been checked out & is o.k., I'd take a close look at rad hoses for soft spots (easy to squeeze) that may be causing hoses to collapse at highway speeds & restricting flow of cooling system. Another thing to try is, turn your heater on high fan & high temperature when it starts to heat up & see if temperature gauge goes down (cooler) at all. If it does, then rad not doing it's job properly.