Vent selection not working After the transmission was rebuilt

You have no engine vacuum to the dash control valve. Recheck your work.

Defrost is default mode. You have an electrical/vacuum/blend door motor problem causing this issue.You need the system checked by a pro to determine the failure

The climate control unit might need changed.

You have a serious mechanical failure in the transmission, it will need to be rebuilt or replaced.

you have a sucked shut or broken vacuum line under the hood somewhere. mine was burnt and broke, same problem. it is a little, skinny, hard vacuum line from left side of motor over to right across the front.

You will need to have the transmission rebuilt or replaced....this is common on the GMs...had to have the same done on my Safari at 125K....the upper bands (most used) goes first.....1st gear is only used for taking off thus has less wear. Good luck.

Usually due to vacuum leak. GMC uses poor quality plastic piping going form the left side front of engine by ac accumulator and runs to the driver side of engine

The HVAC system uses engine vacuum to open and close the blend doors. Check for a broken vacuum line in the engine compartment. It will exits the firewall left (passenger side) of center and will be a hard plastic line. With age and exposure to engine compartment temperatures they become brittle and break. It can be repaired with a rubber vacuum line that fits snugly over the line to reconnect them.

Does your turn signals, high headlight beam work? Also, does the AC function through the dash vents and not the defrost vents?

My cruise control quit and I found it was the vacuum lines running to the servo.

The controls for your heater/vent selection is inside the dash, behind the control switches you use to select the vents the heat/AC will come out.
When you move the selection levers, there's a few things that happen. Vacuum is applied to the doors (which are inside) and they open or they work using cables to move the doors or both. The different doors open, directing air flow to the vents selected.
Remove the controls from the dash (which should just be removing some screws, maybe a few small pieces of dash and pulling it out). The controls will be connected inside so they will not come out very far.
Just check to see if anything is disconnected and try operateing the controls with the heat on to see how they work.
Sometimes they just stick or the doors stick. This system is the way the 1991 models worked but since they are the exact same, it should be this way also.
Look it up in some sort of repair manual for more info. I find with these trucks, they don't heat very well using the front vents anyway. I find using the bottom vents work best for heat unless you need defrost.
The "heater" selection will atomatically use the bottom vents, "bi-level" AC will use the front and bottom vents if I remember right and regular AC will automatically use the front vents. For the trouble in removing these controls (the whole dash would need to be removed for repair) I probably would just live with the way they work if they do work atleast properly (enough heat and AC). If you find that you don't get enough heat or AC when in use, check the cooling system for proper operation and blockages/mineral buildup and check the AC system incase it needs recharging or you have leaks.