Truck will not go into four wheel drive when the slip or abs light is on

The 4 wheel drive will stop working, the computer relies on inputs to help your $X$ but as far as I know if its in $X$ the ABS will not work its programmed that way. Check the 4 wheels and make sure your sensors were not cut that usually happens and will light your abs and traction control light on dash.

Check with a code reader or scanner, the ABS could be at fault

gmc envoy i had done the same wound up replacing all four wheel brake pads and light stayed off

I suggest scanned the ABS module in order to know fault codes stored there; sometimes just need speed wheel sensor adjustemtn, but need the real fault code in there.

get under the vehicle and verify that the mechanic plugged in the abs hall effect sensor. also, did the new unit come with an abs trigger ring?

If you are driving in slick conditions on snow and ice or on wet pavement, this could be NORMAL OPERATION. The lights are coming on to let you know that the Traction Control System (TCS) and the Antilock Brake System (ABS) is actively correcting a wheel slip condition.

If this is happening on dry pavement, and you are not driving like a maniac, (making the wheels slip and slide) then there is possibly a problem with one of your wheel speed sensors that is making your TCS and ABS computers "think" that there is a wheel slipping. You will have to scan your computer system and watch the wheel speed sensor data to find out which one is malfunctioning.

the abs system has nothing to do with it.

Some auto parts stores will read the codes for free, such as AutoZone. Without the code, it's hard to guess. For example, it could be any of the four wheel sensors.

91-97 Land Cruisers had a factory option for front and rear locking differentials. This is a seperate switch and is usually located to the left of US model Land Cruisers. The center dash button locks your transfer case differential changing your Land Cruiser from all wheel drive to four wheel drive depending if you have ABS or not this button either says ECT or Center Dif Lock. If you do not have the Front/Rear locking differential option then your Land Cruiser has open Front and Rear. Hope this answers your question.

Its a mechanical problem related to transmission.You should start from there.