Chevy & GMC Vans

As you may be able to see I had an issue with first gear slamming hard from 1 to 2nd.....after a few months It was starting in second all the time with no 1st at all UNLESS I put it in there manually...then it would bind like the brakes being on. After fluid and filter change etc etc...I had it rebuilt by a reputable guy....I had 1st gear for a but then the same thing happened...no 1st. If I put it there manually (in #1 on the dash) it still seems like its binding a bit. I took it back to him and he says the computer says its shifting fine but I konw its not. My engine light was on due to oxygen sensor...tranny guy says to change it and thats why its running crappy..... Did that..engine light off..still the same. Any suggestions? BTW the van runs and shifts great except its very sluggish on the take off..

What year?

2000
Coming home tonight I went to pass someone and when it kicked into passing gear, it was working good for about 2-3 seconds and started slipping...bad

as well......there are no codes on the dash etc...could there have been something wrong with the housing of the entire tranny? Im stumped and not much of a tranny guy to begin with.

I would like to help you but I too am not much of a transmission internals guy. Sounds to me like the guy who did the rebuild needs to drive it and make it work.