My brother just got a 99 z71 for a winter vehicle. The truck has around 150,000 miles and the tranny was shifting fine, but he decided to change all the fluids to be safe. Well after he flushed the tranny the truck won't go into third or fourth. It shifts fine in first and second.

He has taken the pan off and all the conections seem to be ok. There isn't any check engine lights, but I told him to try and pull any codes anyways and let me know if anything comes up. I did do some searching on here, but couldn't find anything. Can anybody give me some ideas to point him in the right direction or somethings to check

Sounds to me like cluth packs are slipping due to the new fluids.
Did he use regular fluid or swith to a syn blend?
A lot of times, especially with those crappy trans, when you would switch to syn blend or full syn, the tranny would just not want to engage.
I used to have a 4l60 and was pretty good about trans maintaince. Then one day I had a few extra bucks and I put full syn in it. Tranny started slipping alot. It eventually went away for me though

i had a 96 with a 4l60e... i was going down the road and i lost all foreward gears, only had reverse.. i rebuilt it and the parts only ran me $300 for everything, including new planetary gears. after i rebuilt it i filled it with synthetic and all was good till i sold it

I was able to talk to him somemore. He put tran synd back in the truck for fluid. He also said that the tranny will try to got into third at light rpms, but won't catch.