When going up. Medium size hill my cab feels like the heater is on.......both sides of cab heat up.......originally thought cats were clogged and heating up floorboard....but my wife said her side was getting hot too.....cats are on driverside.......maybe heater control freaking out on vacuum drop going uphill?

 

Welcome to the forum

The Blend Door is the temp control on all Rangers(Fords), it covers up or opens up the heater core so air can pass thru it

1995 and up Rangers have an Electric Blend Door so vacuum change wouldn't effect its position.

Going uphill should cause engine temperature to go up, that is normal, and temp gauge on dash should show that.
Normal temp gauge position after full warm up should be just below 1/2 on gauge
On longer hill climb it will go above 1/2 about the same as it was below 1/2, shorter climbs just to 1/2.

In the engine bay there is a heater core bypass valve, it IS vacuum controlled, so uphill vacuum could be the problem.
As a test, after engine is heated up and temp in cab set to COLD feel the heater hoses at the firewall, should be cool
You will see the By-pass valve it will have 4 hoses attached and a Grey Vacuum hose
Unplug the vacuum hose from the valve and feel the heater hoses again
See if they are heating up
If so then yes, valve is OK and loss of vacuum could be the problem.

Follow that grey vacuum line it will go into the firewall.
And you will see a Black vacuum hose next to it, thats the SOURCE of the vacuum, follow it towards the front of the truck, checking for cracks and breaks, this line gets melted by exhaust pipes.
It goes to a black plastic ball(vacuum reservoir) at the lower front corner of the truck.
If the black line has no breaks then you will have to test that vacuum reservoir for leaks