took 2001 blzer [with 4 auto]in for repair on 4wd system. would not go into 4hi also when you went to 4wd auto then tried to go back to 2wd the 4lo would flash and take some doing to get back to 2wd. dealer said most likely the 4wd control modlue after checking the system out. so i put a used one in from salvage yard. same thing. [by the way the afternoon i put the used 4wd control module in it was raining and the connectors/conections were wet. wo i did some caulking] and the dealer did have to take some corrision off the posts. talked to another guy said 8 out of 10 time just the switch so for 39.00 replaced that and not a bit of trouble since. so after 500. at dealer. 150.oo bone yard. 39.00 new switch it finally works. not saying this will work for you, but...

 

What switch did you replace? Im having the same issue where i get 4lo and 2hi though its sluggish to shift but i dont get 4hi. Are you just referring to the dash switch?

 

yes the switch in the dash. [ mine has the 4 buttons 4auto,2wd, 4hi, 4lo]it is real simple to get out just pry. then there are 2 plugs conntecting to the back of it. take care not to bend the holders if you will prying off the connections as they break easy but the switch was bad anyway. again not saying this will fix your problem but after chevy dealer couldnt fix i talked to a certified mech. and he said if the button dont light try the switch 8 out of 10 times that will do it.then if it dont look for the codes? but if you read my 1st post i spent alot of money that really didnt solve the problem wish i had tried switch 1st!!
good luck let me know. if you have the time try online for switch might be cheaper than dealer?