Whats going on when, in a nutshell- The only thing thats holding (for what seems) my 91 Dak transmission together- is the OverDrive Button. When the overdrive is OFF truck sort of seems ok to drive- but when the O.D. is ON...and at about 20 mi/hr, the transmission(?) or something(this is what im trying to figure out)- makes horrible grinding, truck shuttering sensations. I have had it in/out of mechanic shop-he's sure it wasn't tranny, but ultimately couldn't find the problem.

The mech. changed speed sensor unit, e.c.u, trans flush, diagnostics about a dozen times- and slowly but surely the symptoms are getting worse.

 

The OverDrive Unit could be going bad.

 

Sounds like the planetary gear in the trany..