An ******* ran my 66 chevy step side with a late 80’* gmc engine and trans. Low on tranny fluid , smelt so burnt. So i emptied it and dropped pan and put a new filter in then flushed it till wasnt black know more . say its full, jacked the rear wheel up and spins in every gear but one tire starts slower than the other , seems like all the gears are working and shifting smooth but when i take the jacks down and try to put it in gear , it wont move at all, i dont understand. Please help me understand this

Clutch pack is burnt. Needs to be rebuilt.

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I agree with carfixer007 ^^^^^^^

Depending on how it was driven in this condition, one or more clutch packs are likely burned up. Depending on how low the fluid was and for how long, the pump might be trashed and the valve body might be full of clutch garbage and/or metal. The transmission likely needs to be pulled and completely overhauled. That it will turn at all with the wheels off the ground tells me that it is doing its best to engage what'* left in there, but can only achieve a tiny bit of pressure and/or the clutches are so far gone that they barely have any friction when engaged.

As far as the [one tire starts slower than the other] part, that'* normal. From this, I assume you have an open differential. If a differential is built so perfectly that all friction of wheel bearings, carrier bearings, differential pinion gears, and differential side gears were exactly the same, you might see the wheels start rotating at the same time. If you have a limited slip or locking rear end, there might be other problems. Since you haven't reported grinding sounds or other noises from the differential, and you have reported low and burnt fluid, it is likely there is no related issue with the rear end, and the transmission needs to be overhauled.