This does list the 757th and other major allied units involved in the Italian Campaign. This DOES include your grandfather's unit. Yeah! 
http://www.milhist.net/ordbat/mtounits.html
Here's a contact name for all companies of the 757th Tank Bn:
Mr. Warren & Nancy Davis at 1-612-484-8229
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More info:
This site talks about the 752nd and 757th Tanks Bns:
http://www.752ndtank.com/RocketTanks.html
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Please see the Red Bull's site (34th Infantry Div). Your grandfather's unit was attached to them.
Who Were We?
WW II Order of Battle - 34th Infantry Division
Headquarters, 34th Infantry Division
133rd Infantry Regiment
100th Infantry Battalion (Separate) [assigned Naples-Foggia, Anzio, Rome-Arno]
135th Infantry Regiment
168th Infantry Regiment
168th Commandos
442nd Regimental Combat Team [attached Rome-Arno]
442nd Infantry Regiment
100th Infantry Battalion
232nd Engineeer (Combat) Company
442nd Medical Detachment
522nd Field Artillery Battalion (105mm)
34th Division Artillery
34th Division Artillery Headquarters and Headquarters Battery
125th Field Artillery Battalion (105mm)
151st Field Artillery Battalion (105mm)
175th Field Artillery Battalion (105mm)
185th Field Artillery Battalion (155mm)
34th Cavalry Reconnaissance Troop (Mechanized)
109th Engineer (Combat) Battalion
109th Medical Battalion
34th Division Special Troops
Headquarters, Special Troops, 34th Division
34th Infantry Division Headquarters Company
34th Infantry Division Band
Military Police Platoon, 34th Division
34th Counter-Intelligence Detachment [attached]
34th Quartermaster Company
34th Signal Company
734th Ordnance (Light Maintenance) Company
Other Attached Units
Jewish Infantry Brigade (attached Occupation)
A Company, 2nd Chemical Warfare Battalion (attached Naples-Foggia)
35th Quartermaster War Dog Platoon (attached North Apennines)
38th Infantry Scout Dog Platoon (attached Occupation)
72nd Signal Company (Special) (attached Naples-Foggia)
84th Chemical Mortar Battalion (attached North Apennines)
100th Chemical Mortar Battalion (attached North Apennines, Po Valley)
105th AAA Automatic Weapons Battalion (Self-Propelled) (attached Naples-Foggia)
107th Coast Artillery Battalion (AAA Automatic Weapons) Battalion (attached Tunisia)
A Company, 191st Tank Battalion (attached Naples-Foggia)
2nd Battalion, 351st Infantry Regiment (attached North Apennines)
432nd AAA Automatic Weapons Battalion (attached North Apennines)
435th AAA Automatic Weapons Battalion (Self-Propelled) (attached Anzio)
443rd AAA Automatic Weapons Battalion (Self-Propelled) (attached Tunisia)
751st Tank Battalion (attached Tunisia)
752nd Tank Battalion (attached Rome-Arno, North Apennines, Po Valley, Occupation)
753rd Tank Battalion (attached Rome-Arno)
757th Tank Battalion (attached North Apennines)
776th Tank Destroyer Battalion (attached Naples-Foggia)
804th Tank Destroyer Battalion (attached North Apennines)
807th Tank Destroyer Battalion (attached North Apennines)
813th Tank Destroyer Battalion (attached Tunisia)
and still other units which remain to be found, confirmed, and listed here.
Proud Daughter of Walter (Monday) Poniedzialek
540th Engineer Combat Regiment, 2833rd Bn, H&S Co, 4th Platoon
There's "No Bridge Too Far"