Transmission problems in my '98 chev p/u

Hey guys have a problem that someone may
know about. I have a chev P/U 1500 Z71
package and a 1998. My problem is with the
transmission. I noticed that the tranny
fluid was blackish and smelled burnt. So I
drained the fluid installed a new filter and
new fluid. But am still experiencing the
same issue. Now to explain, the truck seems
to slip when going into third and when it
does shift it is hard, also it does not shift
into overdrive. When starting off it shifts
perfect for first and second then it slips
going into third, I raise the rpms and then
release the throttle and then hit the
throttle again quickly it then shifts as i said
hard into third. As my speed rises it does
not shift into overdrive at approx. 100km (i
live in canada) and the rpms raise up to
3500+. I have checked for codes and the only
one that has showed up now is for the MAF
sensor only since i took the speedometer up
to 100kms. I did buy this truck with this
problem and was hoping to get info on
things that i could do to maybe diagnosing
the problem before taking it to a shop (as
we all know that they can be pricey) I am
willing to buy a manual to learn how to
rebuild the tranny but before this hoping
someone might have a better idea. If I dont
have to rebuild it would be awesome as I am
pretty busy with work and all. So this is my
quandry. Can anyone give me some details
on what i can do to diagnose the tranny, or
am I stuck and have to take it in?