transmission and throttle position sensor

Hi folks-
I recently bought a 93 dakota (extended cab, v6, 4x4, auto) to use as a work truck. Needed new shocks in front and brakes, which I put in
without any trouble - it's a nice roomy car to work on. Otherwise it's been pretty dependable engine-wise.
It's started developing an intermittent tranny problem, though. When I got it on the highway after I bought it, I noticed that it tended to shift down into third a lot when going up minor hills. Now it's started to not want ot go into fourth at all. I'll get it every once in a while, but mostly it's driving in third. Bad on gas mileage.
The previous owner said he had the tranny rebuilt a year or so ago. I noticed that the transmission fluid level was very high, so I pumped out a quart or so to get it into the right area on the dipstick. It's made no difference. The button that should disable OD is missing from the dash. The tranny shifts smoothly in every other gear and reverse.
So any thoughts on what might be causing this? I've read that the throttle position sensor can cause problems going into 4th gear. Could this be the trouble for a 93? Or maybe the speed sensor? Any procedure on how to test them? First Dodge - still learning my way around it.
TIA Chris In the Land of Rust (central NY)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I had a simular truck except 2WD. My tranny started to do the same thing around 75k miles except fluid level was fine and changed regularly. It didn't last too long. I was doing 70 down the freeway and it suddenly downshifted to something that made the truck seem like it slammed on the brakes.
The rebuilt tranny has lasted about 100k so far (son owns it now).
FMB (North Mexico)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Have the OD on/off swirch reinstall

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.