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1998 Mazda B4000 - Struggles in town

My 4x4 truck w/ manual transmission and 128,000 miles is fine in town but really struggles when maintaining high speeds ie freeway and going up hills. please help! is this a compression problem?

It could be a LOT of things on a 20 year old vehicle. And I can’t test compression from here on the internet. I’d suggest you have your favorite local mechanic check this thing out.

If you are going to check it our yourself, I’d pull the spark plugs and run a compression check - dry and then wet. Take a good look at the plugs while they are out looking for oil deposits, burnt electrodes ect.

I’d also consider that the catalytic convertor may be plugged.

Is the check engine light on? What codes has it set?

Lots of things could be “aging out” like plug wires, hoses, ect, that proactive replacement may be a good idea but find the problem before throwing a buncha parts at it.

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Doubtful. This is a tough question since you state that it is fine around town.
But start with the basics. Air filter, last changed? Fuel filter if original?
Spark plugs, last changed?

Thanks for the advise. I should add that I have had a tune up recently. I guess the fuel filter would be the cheapest place to start. Thanks

Tune up with new spark plugs and wires recently. I’ll check the fuel filter