Hi guys I wanted to move my rear back
How far can I safely move the rear and back without getting a new driveshaft?

Thank you for your. Input

Mopar or no car

On which car?

69 dart street n strip

How do you plan to move the rear back and to where , the wheel tubs aren't real big

If the front driveshaft yoke isn't supported well in the trans (tailshaft bushing), you'll have problems with that.
I'd be nervous at around 1/2", though I've done 1/4" without issues on a Duster street car.
A-bodies don't have much tail weight as it is, so I prefer moving the front lip of the wheelhouse forward.
Depending on the individual car (they vary, even side/side on same car), you can gain 3/4" - 1 1/4".
I'm assuming we're talking about larger-diameter rear tires & interference at the traditional spot.
Also, look in the stock tubs at the contours that can interfere...

Interesting question, so just how much engagement is needed for the yoke into the transmission?
I see people say, push all the way in and pull out 1”.
So is 2” too much to pull out, just where is too much?

Merry Christmas Moparts ! Hi , Are you using the Factory Bellows Ballon TailShaft Seal ? or one of the Aftermarket Slim seals ? Makes a Difference in seal to yoke , But the measurement is from the Case End , to Centerline of U-Joint . On A-Body's and F-Body's it's 3.50" , and on B-Body's and E-Body's it's 3.750" .

Ok, but the question is, how much deviation is acceptable

I think Mopar FSM recommend pushing the tranny yoke all the way in and then pull it out .750, 3/4 inch, with all the cars weight on the tires for center to center measuring: scope:
I've used that method on a lot of Mopars cars for years with no regrets

Can't be answered with given information.

May have anywhere from very deep to very shallow engagement to start.

No idea why you want to move the rear end.

Put it where you want then share how much slip yoke engagement you have.

My max would be 1". That's with one of the long factory yokes. If I'm building a shaft, 3/4, max. Need to take suspension type and anticipated squat or separation into consideration.

Moving a rear back is the wrong direction BTW. Maybe on a street car if you're trying to avoid cutting the wheel opening. Maybe.

The inside bore of factory slip yokes isn't splined all the way, the first inch or so isn't splined, so the actual spline engagement is less than it appears from the outside. I wouldn't extend the yoke more than the factory dimension.