4L60E Shifts fine in 1st, 2nd, & 3rd but will not shift into OD

You more than likely need to have the transmission overhauled

Check the fluid level. You may want to consider having the transmission flushed. (I recommend BG Products flush system.) Properly flushing the trans removes any varnish the may have built up causing erratic behavior.

there is actuators for each gear i would look into testing those

Sounds like your transmission is in limp mode. This is a fault mode in which the transmission either loses electrical power or the PCM deactivates all of the electronics. The transmission will default to max line pressure. The driver will have Reverse and 3rd, with manual 2nd available by putting the gearshift in 2 or 1 (doesn't matter). 1st, 4th, and TCC lockup will not be available. It will not hurt the car to drive it like this for short periods, but use manual 2nd to get the car moving, and be aware that the trans will generate more heat than normal while operating in limp-home mode due to increased torque converter slippage during 2nd gear starts and no lockup in 3rd gear.

The other from what you described, trans shifts 2nd to 3rd by itself in D or OD and locks the converter : ShiftA solenoid failed. Easy fix in the pan. Or a wiring problem from PCM to trans.. or PCM.

Thank you for using fixya and have your Safari scanned for the DTC codes that would indicate what issues your transmission is having to put you in to limp mode.

a number of issues. Tcc regulator bore worn out, the infamous check ball seat it hammered badly, or/and calibration issues. I have seen all three issues on the same trans - often!

Did you change the transmission filter and gasket as well as the transmission oil before running your truck.

Sounds like you have a bad shift solenoid. Not sure if the Chevys are a solenoid pack, or seperate, but it will need to be replaced.

The EMC detected and problem so it's in limp mode, As long as it's in limp mode the 4th and OD will not activate. Take care not to burn up the transmission by running it hard while your in the limp mode.
Your going to have to get the truck scanned to clear any codes and why there are DTC codes. Auto Zone scan for free and most auto shops charge $10.00 to $20.00 to scan sine they want the business.
If you want to clear the codes your self you can put the ignition to the run position and pull the ECM (Batt) fuse out for 20 sec and place it back in then start the truck. It may stall but let it, just start it again and this would have cleared all the DTC code history.
The transmission should be running fine unless there is a problem and the service engine light will be on again and you'll be in limp mode still with an updated DTC in the ECM.
Good luck and hope this helps.

do u have any trouble codes for the trans? u need to find out to see if a shift solenoid (possible 2-3) has failed.

Does it just not shift up to 3rd or when it shifts to 3rd it is just like neutral?