While driving my truck wouldn't accelerate over 20 miles, what would cause this?

Have an electronic evaluation carried out, it's quicker and in the long run safes you frustration and cash.

could be crankshaft sensor or fuel filter,check air filter.

You can have very bad COP Ignition Coils without a Mil
Lamp on dash

You can also run Mode #06 on professional scan tool software,
to look at cylinder misfire counts,but need to have someone
drive it, while you look at data.

The EGR Valve or other egr items, could cause the uphill
thing, as your in a high vacuum mode & the coils will break
down as well

I wouldn't throw around the word diagnostic, as your only
looking for stored data, Generic or MFG --with a code reader
or professional scanner,that is no big deal or any computer
involved

O2 Sensors get replaced every 100,000 miles or so,&
yours may have never been done

You obviously need to start will new spark plugs,OEM
config,not some fancy thing with prongs,they won't work
in most fords.

Make sure there is no vacuum leaks anywhere including
exhaust leaks before the converter.

Check PCV Systems is tight & works

Check TPS Voltages

Test,don't replace anything,ever,have to fail a test to
need new parts,except plugs,O2,gas & air filter.

check for oil in the air cleaner, the smoke is most likely coming from the engine compartment and it will come out of the back of the car when you are driving, can be caused by blow by or can be oil burning off on the engine from a leak

Ignition Wires on wrong

Worn out Cats --can't test?
140,000 miles & 11 years--replace them

Check Fuel pressure

Front O2 Sensors--not the issue, but get
changed at 100,000 miles

I've had this happen more than once and it always turned out the the timing belt was off by one tooth.
I bought a used car in the 80s that had had its belt improperly replaced, told the dealer that the car had a problem, wouldn't tell them what was wrong and after ~ 20 minutes, they dropped the price to one I had offered, saving 34% off their price.I brought a couple of wrenches with me when I picked it up and drove two blocks and corrected the belt in <10 minutes.I drove it for 10 years and 150K miles before selling it.
Have the belt checked again.

The average life of a taurus or sable transaxles is a
surprising 65,000 miles,then you have to rebuild it
I only recently read that

In the early 1990's I owned a trans repair business
& over half my work was taurus & caravans
The rest was japanese

I can not see that being the original transaxle from
1995, but I wouldn't hold out hope of any type of
simple fix

There are non to my recall ,other than a cable on
the throttle body & I don't even know if they still exist

1 quart of oil in 1,000 Mi. isn't out of the range of normal for an older car. If it's used for mainly stop & go type driving [ city driving ] it's about normal. As things wear inside the engine some oil will find it's way into the combustion cylinders and burn with the fuel mixture, the catalitic converters will clean up the exhaust so you wouldn't see any smoke from this small amount being burnt.
A clogged air filter or faulty PCV valve will cause a higher rate of oil consumption, if those are good, I wouldn't worry too much about it. Just keep checking the level.

your fuel filter, injectors and fuel regulator should all be checked-these clowns (sorry, mechanics) should have checked line pressure FIRST, and I might have suggested that the fuel tank be removed and cleaned after it failed the first time-n telling what is in the lines, and injectors are easy to clog. Since this is an older car, and more difficult to diagnose, it might be a good idea to look for a Volvo specialist that will diagnose the problem rather than throw parts at it.

Check the front drive axles and boots. A bad cv (constant velocity) joint can cause this.