Locate the 3 wire plug going into the front of the transmission, should have a blue, red, and white wire. Blue is Reverse, Red is Forward, and White is the Hi/LO range. 12V is supplied to these wires to select a function. If 12V is not present at the white wire, the transmission defaults to low range.
Measure between each of the 3 pins and the transmission case, should read about 9.5-10 Ohms. If no contiunuity, remove the plate covering the solenoids and insoect the spring plate that grounds them.
The three solenoids are the same, and can be swapped around to troubleshoot if need be.