My 1999 Chevrolet Silverado LS 4.8 liter has problems

I put a second hand steering column in
it. It was a back door mechanic that put
my steering column in and also my
transmission in. He didn't put all screws
bolts in it. It's done now by professional
mechanics and I had my trucker parked
for 3 years and it took me a while to buy
those two items is why but that's
besides the point. Just giving you all the
facts. So after I got it out the shop it
was fine that day but they told me it was
a quart too much in the transmission. I
put it in shop because I my rear oil seal
was leaking and I was putting 5 quarts in
it lasted 2 days so I put it in shop. Now
the transmission when I get on road
won't catch speed, when you put it in
another gear it won't catch gears in my
shifter. Answers are all in these
questions but I'm not a professional
mechanic so I was hoping you could
help.