89 Eliminator not shifting from second to third

[Virginia]

Unless I let up on the gas. I've been told to check the vacuum modulator on the transmission but can't find a picture of what it looks like. I have a switch on the dash that says power or economy so have been told that it's an electronic transmission.

 

The fluid is good. Any suggestions?

[MuddFoot]

i had this happen on my xj i did a fluid and filter change and it fixed it. when i had the pan off i checked the solenoids. the 96 xj did it to ended up changing the tcm  

[Virginia]

I'll try that.

[Virginia]

Turns out it was a stuffed up muffler and bad catalytic converter.  This truck had been sitting in a field for a while.  Apparently, critters got up in there.