1995 351 motor problems

New to post but I have read the site for a while now.

I have a 1995 XLT with the 351. Only has 112,000 miles on the motor.

The thing runs awesome and fast on the street and in the trails but when I am going up a large grade the motor bogs down and starts to make a knock sound and even sounds like it's backfiring through the intake.

So far I have changed the plugs, added a K&N filter kit, Flowmaster, Throttle body spacer and a MSD coil.

Like I said, It runs awesome all the time EXCELT for the grades.
This weekend while out in the desert I had it in 4high and in first gear going through some sand and it boged down and started that backfire thing.

anyone have a clue what could cause this? I just dont have any power at low end on grades or off road. When I am going slow and crawling I have a ton.

Stock size tires (31's) and 3.73 gearing.

No check engine light that comes on when this happens.

Someone help me?
Thank you

Also like to add that I have tried putting 91 fuel and also 87 and it does the same thing no matter what gas I put in it.

HEY JUST A THOUGHT FOR YA TAKE IT IN TO A DEALERSHIP OR A FRD QUICKLANE AND HAVE THEM RUN A POWER INJECTION FLUSH, AND SCOPE AND SCAN YOUR ECU OH AND WHEN WAS THE LAST TIME U HAD YOUR TRANS SERVICED

sounds like timing could be the culprit.

You might want to check your distributor for worn out bearings. I've seen distributor shaft play cause your problem. Just a thought

Treader