I have a 1988 D-50 4x4 with a rebuilt 2.6 motor, It does not run well on small grades unless I pull the hose off from the air cleaner to the back valve cover, When it gets air it runs strong.
Had a similar problem with a Isuzu pup, anyway I found a mouse nest in the fresh air intake tube was restricting the air when it was under a more than normal load. Ran fine if you didn't want to go above 30 and wasn't in a hurry to get there.
We have checked everything for blockages, The motor was removed and overhauled a few weeks ago, It was doing this before the overhaul though but passed smog easily, If I remove the top of the air cleaner it runs very well, I am only getting about a 100 miles to a tank of gas also.
Cut the top of the air cleaner off or at least put 1 inch holes in it.....it will fix it.......my pal dud this years ago. I have the truck gets 33 mpg! Not bad for a clunker !
I found out tjat I had a bad carb, I had a new one installed and it fixed the problem. 2015
I cut holes in top of air cleaner....runs great! 88 dodge ram Carolina . Gets 33 miles per gallon:-)