how do i know its the rear end is the problem

Im going 50mph, I stop at a red light with some trouble, as the car is sitting the engine revs as if its still going 50mph, the car trys pulling forward by itself. the car revs loud then it stops then again revs loud...it stops. not the engine but the reving. I go to a person who fixes transmissions they say its the transmission, I go to a person that fixes engines they say its the engine. they just have me going in circles spending money i dont have. I already put a engine from a different car and transmission,they have 30,000miles. but its still the same. dont know what else to do :headshake.

When you had the engine replaced, did you check for vacuum leaks? The IAC valve is also known to stick open sometimes and cause this. It is located on the back of the intake plenum and can be easily removed.

If the engine and transmission have been replaced an there isn't an intake issue (leak or bad IAC) than it may need a new PCM. You can pick one up at a junkyard for cheap usually. Just make sure it matches the 4-character code of your current PCM.

What year is this Mustang? Is it even a Mustang at all? Does it have cruise control? Were you using it? What engine and transmission does it have?

Nowhere near enough information to make an educated guess here.