1990 pickup truck 2wd with the 4 cylinder 22RE 4 speed tranny.

Fired up no problem, but when we put any kind of load on it, it would die, only drove in reverse. Rev's up fine.

Any clues as to what this could be? Sensor??

 

Nobody knows???

 

Your engine needs fuel and spark to run, it sounds like your ignition system is operating properly, check it, then move on to the fuel system, starting with your fuel filter and fuel pressure to see if you fuel pump is working good, then go on down the line checking every thing between your fuel filter to the injector.
I think it sounds like it's starving for fuel, but there are lots of things that need to be checked seeing as that engine is fuel injected and controlled by a computer. My boat engine was having the same issues, and it was the fuel filter, it would start up fine and you could rev it up, but you put a load on it and it couldn't keep up and would not have any power.