I have a 1976 GMC C-15 with a 454 and 3 speed 400 turbo tranny (both factory) with about 175k miles, though both engine and tranny have had some upgrades/rebuilds done to them (I'm boasting 400HP! :) ). I recently replaced the starter and solenoid (like two days ago), but I don't think it is part of my issue. Everything works great when the shifter is in 1st or 2nd or Reverse gears... when it's in Drive is the issue.
While in Drive my truck will take a few extra moments to shift into 2nd gear (from a stop) and will shift very hard when it does and is immediately followed by a shaking and strange clicking sound (constant, but seems to go away while accelerating, and definitely coming from the tranny area).... it almost sounds like it's a diesel. Now keep in mind this doesn't happen if I leave the shifter position in 1st or 2nd or Reverse.. so it's probably not the actual gears.
My flywheel isnt missing any teeth, nor is it loose. I'm pretty sure my starter is properly shimmed and I even tested to make sure it's the tranny by starting the truck then dropping the starter and trying to Drive it again... same result.
I also changed the transmission filter/fluid recently (less than a year ago). My best guess is the cluster gear... I really don't have a dime to my name so I need to make whatever investment I commit to be the correct one.
One more note to add: the transmission has been slipping at high RPM for a while now. I knew I'd have a serious problem sooner or later. Perhaps it's time for a whole new tranny?