Okay, lets itemize what we have just heard.
1. Engine under load (in gear) does not go above 1200 rpm. There is no code according to the poster. What can cause the engine to run poorly and not throw a code?
Primarily what affects fuel the most? This to me sounds like fuel starvation, because of point 2.
2. Engine under no load (in park) does rev up and according to the poster revs fine at higher RPM. There is a key element here that must be realized. Something is causing the engine to run leaner. Spinning in neutral or park requires significantly less fuel and air than in a loaded situation. It also produces less exhaust.
3. Able to move it in 4lo only up an incline, in reverse.
So, the first question. Does the thread starter have a FSM?
Symptoms would seem to suggest to (Just an engine technician, not a toyota guy nor terribly familiar with FZJ80 engines just yet, but do have a manual...and I did happen to stay at a Holiday Inn Express recently) check the sensors on the intake. That would be the mass air sensor, the intake temp sensor. These two tend not to throw a code unless they go grievously out of range, or short to ground. The example would be that your truck thinks it is 100 degrees outside and really it is 40 degrees.
Other points that I could think up while on the brain storm is the throttle position sensor may have a flat spot in it. It requires little fuel to rev an unloaded engine, but much more pedal to keep a loaded engine moving under load.
I suggest you find your voltmeter and start taking readings off the sensors that determine air/fuel mixture.
Every sensor has an operation range that it can stay inside but sometimes even in the range there can be a problem. The example above about the temperature being too low or too high compared to actual. Mass air flow not showing air entering the intake at the proper velocity. TPS sensor not showing the correct throttle position but not outside the range of operation or shorted.
There is also a question about the condition of the intake itself. How does the filter element look? Have insects/critters setup a colony in the air filter? How about where the filter enters the fender? Mud or critters build up in the entrance?
How is your battery voltage to the engine, from the alternator, and what do your grounds look like? This could be as simple as a loose groundwire. First rule when trouble shooting a nightmare like this.
This should throw a code, but does your neutral switch function properly? Your engine uses this signal for air/fuel ratio.
What about the igniter/distributor? when was the last time you had your rotor inspected?
Do you know how to check the ingition coil with an ohm-meter? If you have a short to ground on the primary or secondary side this could cause you several runability problems and might not throw a code.
Okay....time for a beer. These were just the brainstorms I could think of, if I am out of my mind. Let me know, I am open to constructive input. I enjoy troubleshooting my own stuff a lot. I hope some of what I may have said gives you some ideas.
