my question is exactly what the title says, will a 92 toyota pickup axle shaft fit into a 84 pickup?

 

No...

 

the 92 is longer they made the rear ends wider in 86 you can use one from 79 through 85

 

Yours is a solid axle front. As was stated, 79-85 solid axle parts work for you. In 86 they went to IFS front (in the States) so to match the necessary width of the new IFS fronts, they went 3" wider for the rear diff. It's a common upgrade for us early solid axle guys. IFS rear diff assembly offers more stability and bigger drum brakes. In my case, everything bolted into my 84. Just get the IFS leaf spring U-bolts if you get one from a yard. The diff tubes are a bit larger in diameter.