Need help (In regards to do not shift light)

[trautoranch]

Have a 86PT38 8V92. The do not shift light is on and it will not shift
out of neutral. Have check fluid, ground wires and many other things.
Did notice that the tachometer is showing the engine to be running
between 1000 and 2100 rpm's. When i turn the high idle on it shows the
engine running 1500 to 1700 rpm's. I turn it off and the engine slows
back down to normal and the tachometer doesn't. Could this be giving a
false signal to the transmission and causing the do not shift light to
be on. If so were do i start looking for the problem. Any help would be
appreciated.
Best Regards,
Rick Mcintire
86PT38
Patillo,Tx