99 f350 7.3 auto. Had transmission rebuilt in november. March I was towing a trailer 65 mph and the truck shook for a second, Felt like i hit the rumble strips but I didnt. It did this again towing a trailer but slower speed about 3 months later. July it started shaking every time I tow a trailer. But only when trans temp hits 180 degrees and only in 1st and 2nd gear. If I pull out like a grandpa it will not do it. One time the trans over heated. The trans guy had it apart twice. !st time He said he had a gasket in wrong, and replaced it and the clutches. Said he was sorry a bunch of times, I figured it was fixed. But nope, still does it just does not over heat. 2nd time he took it apart he said the sprag wasnt holding and blamed it on junk chinese parts. But It still shakes. THe whole truck shakes bad, spill your coffee bad but only pulling out in 1st and when it shifts to 2nd.Any faster than that its fine. Unhook the trailer and its normal. Trans guy did not replace the torque convertor ??? I was thinking of putting one in but hate to spend the $ if it doesnt need one. He says its not in the trans, run it till it breaks. Im not letting him touch it again.
Since this problem I have replaced the carrier bearing, rear leaf springs and shackles. U joints are good.
Under 180 degree trans temp with my 28 ft camper hooked to it, pull out hard and it will spin and take off, no shake.

I would say it’s the torque converter. I had an issue with my 7.3 doing the same thing but it was the flywheel ford used in the 5 speed, installed a solid flywheel and issue solved. You could also check the transmission mount sometimes the bushing on the mount wears out. Eric

Normally you get a shutter it's the torque converter.i had a tranny guy tell me if your having Trans problems use a Gm product put it in the tranny drive it up and down the road shifting thru the gears several times bring it back in change the fluid and filter and it takes care of over ninty per cent of the problems.hes done it in all makes of. Cars and trucks.if you want the info private message me.

I would say its not a shutter. Its a shake that makes you wonder when the driveshaft is going to fly out.

check the ground,,,, That fixed my dodge but it was a 2000

Could be low on brake fluid..
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Trans mount is good. Blinker fluid is at full mark.

the negative ground for DC voltage ......truck has a ICU,,,, ICU controls stuff ........Block heads................
Almost like taking to sponge bob