I have 97 4x4 f-150 V6 4.2 litre that in 2wd tries to go into 4wd causing a grinding then a thump. When I put the truck into 4wd there is no grinding or thumping.
I believe one of the vacuum solenoids could be bad. Just was wondering which one would cause it to want to go into 4wd while in 2wd.
The left most solenoid looking under the hood is the 4wd and the other with the pink vacuum is the 2wd.
I have no vacuum from the solenoids but from the lines going into them.
There should always be vacuum on the colored line leading from one or the other solenoid. The vacuum keeps it "drawn" into whichever mode is selected. Your diagnosis is correct and the 2WD solenoid should be bad. You know of course that these solenoids only affect the front axle being engaged or disengaged and has nothing to do with it actually being in 2 or 4 wheel drive.
I had a lot of problems with my axle vacuum system when I first got my '98. Took a while but did get it straightened out. My guess is that the 2WD solenoid goes bad because that's the one activated most all of the time. I picked one up at the salvage yard off of an Expedition. If you go that route grab the lines too since they sometimes crack/break.