Here's the quick and simple;
My 4l60E will not shift into 3/4 in drive mode. I can manually start from a stop in 1,2 and 3rd gear but cant get 3rd to go over 20MPH and it feels like it wants to shift but doesn't happen. In drive it will shift fine 1/2 but the second it should shift into 3rd I get high RPM and no shift. Reverse is fine. About 10 months ago I had the same issue and just replaced the transmission with a used one...problem solved. This week I have replaced ALL the Solenoids in the valve body, flushed the fluid, checked fuses and ran individual ground wires from the ECM and TCM. The transmission DID NOT code until after I replaced the Solenoids. Now I get a P0700 and P2435 code. The problem is still there. Do I pull the pan and replace the Solenoids again or check to see if there's a loose wire in the harness? I don't have the money to take to a transmission shop and I really don't want to have to replace the transmission again as that is a total pain in the a** and pricey for the tranny. There were and are zero leaks and the fluid is topped off.

 

Almost sounds like a broken forward sprag but you should have no 3 and D should do nothing. Only 1, 2 and R should work.
Could be burned up 3-4 clutch pack. Was fluid really dark and smelled burnt?
If this is happening exactly the same as before, it sounds like something else is going on. Is the shift linkage properly adjusted?
Any chafed wired going to the trans?
Throttle position sensor / Throttle body bad?
Was the torque converter replaced when the trans was replaced?

 

Definitely a broken/slipping forward sprag. I've done a bunch of them. Also, there is a GM service bulletin that addresses the exact concern of No Movement When Transmission is Shifted to Drive or Third - Normal Operation When Shifted to Second, First or Reverse (Replace Forward Sprag Assembly)