Leak the trans leaking at the tail section.how to stop the leak.

What year ?

there are turbine speed sensors that are located in the area you mentioned
if they are faulty ,the TCM has difficulty in correctly changing gears

one of two reasons:
1- the part of the shaft which comes in contact with the lip of the seal has a groove in it
2- there is a brass bushing behind the seal in the tail of the trans which is meant to hold the shaft steady; if it is worn, then the shaft is moving out of range of the seals' ability to remain in contact with it.

You have a leaking trans output seal leaking into transfercase adapter or trans tail, need to remove transfercase to replace and order seal from a dealer they are hard to find any were else. good luck

Pull the vacuum line of the transmission modulator valve. If there's oil in the line, the diaphragm in the modulator valve is cracked and engine vacuum is pulling trans fluid into the engine intakee

please clarify the question

you will still need add the proper type fluid to proper fill level, get under the truck, clean up transmission so you can visually monitor where the leak is coming from. Chock the wheels, emergency brake set, start and idle vehicle check for leaks. If trans is leaking it will probably be a front seal (oil will drip from small hole in the bottom of bell housing), pan gasket, loose cooling line to radiator, or rear tail shaft leak ( where drive shaft enters the trans.) Don't forget to check the lines for pin holes and the radiator for leaks.

No there is no internal oil seal in the tail housing.
Maybe you damaged the seal when refitting the tailshaft.. It is easy to do.. Did you grease the seal so the tailshaft wouldn't damage it upon reinsertion?

If the van shifts ok when cold but hard when hot then the pressure control valve in the trans is bad. When the fluid temp climbs past a certain point the PCS sensor shorts out and stops controlling fluid pressure. This usually causes a whine from the trans (pump) and hard shifting. This is because the PCS can no longer control the pump pressure so the pump now runs at full pressure and shifts as if you were using full thottle all the time - the opposite of what your doing, which is to feather the throttle trying to ease the shift. So now the trans calls for a full pressure shift when your using part throttle and it slams. The (PCS) part is cheap enough but the trans has to be removed to replace it and that's not cheap.

Give this a shot, it often works and it's cheap enough to try.

First, have a good mech carefully check the rediator trans cooler section to be sure there's no leakage of coolant from the enging to the trans coller section of the radiato. This is important!, if antifreeze is leaking into the trans fluid via the cooler it'll cause problems that are otherwise impossible to diagnose and eventually complete trans/torque converter failure.

If there's no radiator problem then power flush the trans and change the filter, then install an aftermarket trans cooler in series with the built in cooler (the instructions with the cooler detail this). It's about $60 for the cooler and maybe one hour to install it.

This whole process will drop the trans fluid temperature 40-50 degrees and often that's enough to get back into a temperature range that will allow the PCS to work reliably again.

If it doesn't work your not out much, if it does work you'll probably get another couple of years out of it.

Al

You have a Seal out in the Tail of the Tranny. Where the Drive Shaft Yolk Enters there is a Seal that goes around it to Prevent this.