"D" or "OD"
 

HMMMMMM. im guessing you mean it stumbles while say your cruising 55 mph in 4th gear? My guess is you have carb issues and its not liking the low rpm of the 4th gear much. You said you have to pump the pedal....I say its the carb needing rebuilt.

well if its while cruising it could be a torque converter issue or a TV cable issue,

i had a beater s10 that had a similar issue, you can never just cruise with it, always had to be down a gear and have some rpms up. ended up being a issue with the accelerator pump in the carburetor for whatever reason it was allowing excess gas into the engine. just something to look into

read what abig84 said below you.
Exactly. I had a carb'd truck with od and it hated the low rpm's until i had the carb rebuilt.

Im just giving other possibilities no need to be like that. I work at a shop and its not always the common thing that make a problem happen.

Yea,those could be the problem too. and by the way,i didnt reply to you in a "douche bag" manner. No disrespect intended at all :icon_wink:

if it drives good in drive,go for it. you will just have some crappy gas mileage. but some awesome throttle response. lol but if its stumbling in od, and its the carb,one day or another its gonna do it in drive....id start calling around some old corner carb shops and see about a carb rebuild.

I use drive when i tow my trailer

gives me a lot of rear end torque and feels like theres an engine brake going down hills

Like the big rigs have !