All right I've had two trucks now that I've had this problem with they seem to lose power on the highway and when I'm trying to go over around or over 50 mph I have to actually downshift gears sometimes to try and get the truck to gain momentum and both trucks lose a lot of power and speed traveling uphill My 86 ranger would run rough at idle and every time I would use the code reader on either truck I would get code 31 or 34 My 86 ranger that is EFI would shake real real bad at idle I would lose power when I got to 4th or 5th gear. My 89 has a DIS set up and it runs fine as long as it's 4th gear but once I get up to the highway and get up to speed it wants to lose power. My 89 does not have a Cadillac converter on it so it does backfire could this be a result of my problem my 86 I bought an aftermarket Catalytic converter for it and then shortly after that I started having the issues I mentioned earlier. is this something mechanical or is it something with the timing maybe or what I just don't know what to look for please help!!

 

Not much to go on

I would start with testing Vacuum on the engines, $15 vacuum gauge can tell you A LOT about any gasoline engine

17-21" steady vacuum at idle, lower means low compression, low compression is low power
Raise RPMs to about 2,500 and hold steady, vacuum will go down and then come back up and hold steady, if its slowly dropping you have clogged exhaust, if its slowing going up you have clogged intake(air filter, maybe)

Change fuel filters, could be reduced fuel flow at high demand

Change gas stations, could be poor fuel quality