What would cause it not to go in 2 wheel drive?

There are a few potential issues that could be causing your 2004 GMC Envoy SLT to go into 4-low but not 4-high or 2-wheel drive. Some possible causes include:

1. Transfer case issues: The transfer case is responsible for controlling the engagement of the different drive modes. If the transfer case is damaged or malfunctioning, it may not be able to engage 4-high or 2-wheel drive.
2. Shift motor issues: The shift motor is responsible for controlling the engagement of the transfer case. If the shift motor is damaged or malfunctioning, it may not be able to engage 4-high or 2-wheel drive.
3. Vacuum issues: The vacuum system is responsible for controlling the engagement of the transfer case. If there is a problem with the vacuum system, it may not be able to engage 4-high or 2-wheel drive.
4. Wiring issues: The wiring that controls the transfer case and shift motor can become damaged or corroded over time. This can cause the transfer case to not engage 4-high or 2-wheel drive.
5. Control module issues: The control module that controls the transfer case and shift motor can fail. This can cause the transfer case to not engage 4-high or 2-wheel drive.

It's best to diagnose the issue by a professional mechanic, who can perform a proper diagnosis and repair the issue. They will be able to determine the specific cause of the problem and recommend the appropriate repairs.

If the shaft is powering out to rear end then internal on rear possibly broken differential. If no power from case to rear then broken in case.

It is not the ABS causing this problem.

It sound like your trying to turn on black top or concrete. Limited slip four wheel drive trucks,car etc should not be drivin on a hard syrfice with the 4 wheel drive engaged it will drag the 2 wheels when turning and cause the wheel to hop.Depending on the direction your turning 2 wheels will travel faster than the others.

Just to confirm, you don't have an electrical 4WD selector; you have to move a shifter on the floor of our cab? If that's the case, I'd suspect that you have a linkage misalignment or malfunction that doesn't allow the 4WD selector to completely return to the 2WD position. Putting the truck into reverse and back to drive probably causes a jolt to the 4WD linkage which disengages the transfer case from the driveline. Check for slack or binding in the transfer case shift level linkage (clean and lube the linkage won't hurt).

it is most likely an automatic awd, 4 hi is full time 4 wheel drive, 4 lo is full time 4 wheel drive but at a very low gear ratio and should not exceed 25 mph or could cause damage. and DO NOT shift into 4 lo while vehicle is in motion. Auto is a 2 wheel drive but automatically engages into 4 wheel drive under heavy acceleration.

Have your right front Constant Velocity (c.v ) Driveshaft check out. Noise goes away on 2 wheel drive is there is no torq on the shaft like there is on 4 wheel drive. Bad c.v.shaft. Hope this was very helpful.

look on fire wall pass. side you will see 2 parts with vacume lines attached 99 percent of the time that is what went i had the same problem

the electronics is malfunctioning for the 4 wheel drive. if the button is sticking in-between 2-4 wheel drive it could cause this problem. have fluid in the transfer case checked because if that is low it could also cause it.

Many times this type of issue is caused by the motor that changes the transfer from 2 to 4 wheel drive, they get stuck, another issue is dirty fluid, have you had the transfwer case serviced?