I like the tool. I think it should apply enought tension. Can you rotate or vibrate (impact hammer or even palm sander) while lifting? This might help "saw" off the crud if that is what is holding it in. I was amazed when after spending 5 hours trying to get #4 GP out how easily I managed it once I made a "tool" that applied pressure while I could simultaneously rotate it. It was basically a technique I finally remembered from messing with swollen GPs on the 6.5 GM diesels - spin And pull....

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2006 Liberty CRD - the commuter. GDE Full Tq Eco, Samcos, etecno GPs

2006 GMC Sierra Duramax/Allison - the toy
RIP - 1995 Tahoe 6.5 TD
1990 JD 332, yep, the mower is a diesel too

I tried using a multi-tool on it at the same time to vibrate it.

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2005 Jeep Liberty CRD Sport - GDE Full Torque Eco-Tune and TCM tune.
1990 Jeep Wrangler - Some parts are still Jeep anyway.

darn. Okay, how about tension and time? I cannot tell where the lever-arm end of your tool falls, but could you get a jack under it? Maybe a turnbuckle with bolts instead of hooks or eyes and just give it a turn every couple hours and let it sit?

On edit
Darn.....

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2006 Liberty CRD - the commuter. GDE Full Tq Eco, Samcos, etecno GPs

2006 GMC Sierra Duramax/Allison - the toy
RIP - 1995 Tahoe 6.5 TD
1990 JD 332, yep, the mower is a diesel too