Transmission problems

I have an 1995 chevy astro van. I bought is
about two months ago. It has about 199
plus thousand miles on it. When I first got
the van it shifted fine. But in the past month
the shifting has been concerning me.
Usually when I start my vehicle I allow it to
warm up a min or two before driving. So as I
take off then you can feel a slight and hardly
noticeable shift in the transmission as it's
changing gears and as your driving. But as
I'm driving after a couple of minutes the the
shifting changes. I can't fell the slightly shift.
Before my vehicle gets up to speed on the
take off. Before my vehicle just simply goes
through the gears as normal. But like I said
after a couple minutes then it changes feel.
As I'm stopped at a light or stop sign, I'll
start to go again and it takes more
acceleration to get up to speed and I can
feel a thing as its going through its gears.
I'm sure it shifts but it takes more power and
acceleration and I can't feel the shift. It just
goes from 0 to 60 with no feel in the shifting
and the engine is louder then when I can feel
the shift. It feels like I'm driving in overdrive
all the time. So my question is. How do I fix
this problem without buying another
transmission ?  Another thing I noticed, my
gauges are. P. R. N. Overdrive. D. 2. 1. Is it
possible my gears are messed up to the
point it gets shift into overdrive while I'm
driving and remains there. ? I noticed also as
I'm going through my gears as I shift from
park to number 1 on my gear box. I can only
count 6 out of the 7 total gears I have on the
gauges. My overdrive is right next to drive
and I can't seem to put it in drive. It keeps it
on overdrive I guess and that might be the
problem. Idk. But I do know the my gear
shafts is pretty loose between the overdrive
and the drive gears. Can someone help me
solve this major issue. If it's major even ?