I have a 98 awd astro van , is there a way to Change the torque ratio between the front and back wheels

what was your test to determine the failure ?
if you jacked up one end and the car didn't drive off the jacks , that is how an all wheel drive works
It has a differential in the transmission that allows the front axle to drive at different speeds to the rear ( same as one wheel on the axle will go faster than the other when going around corners)
it is used to prevent torque wind up in the drive train and destroying transmissions , diffs and drive lines
it has to have all 4 wheels on the ground so that the 3rd diff is not operational unless going over a speed bump or dips in the road
however if you have a 4 wheel lock for the transmission, that is different as it locks up the 3rd diff in the transmission making the car a 4 x 4

otad doesn't help ---did the trans come out of an 05 astro van s-10 blazer /jimmy s-15 or safari and it your awd because you need to match the rwd with an rwd and a awd with an awd /4wd from above and the unit has to be compatible with the 05

Could be your has been replaced before.I do a lot of different vehicles, so bear with me. All joints are basically pressed into the spindle with the nut. Hammer on the spindle at the ball joint to get them to release. Some joints are bolted on, some are riveted on (grind, heat and beat them out) to the lower control arm some are pressed in. If yours is bolted on the lower control arm, remove the bolts. Knock the ball joint out of the control, replace it. Consider yourself lucky.

What makes a car AWD is the fact that there is a diff in the transfer case in the gear box . This allows the front diff and the back diff to operate at different speeds from the road surface just as one wheel can go faster in turns than the other side.. To say that the AWD is not working would mean that it is not going any where as the rear or the front will have no drive because the diff will not be operating as a diff.

i have a 99 astro awd auto 4.3 what is the number of clutches it should have inthe transfer case6

I always take the tire off and go through the wheel well. The middle one is the toughest because the steering column is in the way..

i am not mechanic but realise logic control senses excessive rear wheel spin and transfers some torque to front wheels. try quick take off on gravel road to see which wheels dig in or have friend spot if front wheels are driving when they should be. could be simple corrosian at wiring harness. unplug and spray with contact cleaner and re connect and re try. this wont cost you anything.

Could be low on oil, trans doesnt have fluid, wheel bearings are gone, or could be something stuck under the vehicle and either the tires or the drive axle is rubbing against it.

same problem with my 98 astro awd they(alighnment shop) did the knock out and solved the problem also check for idler arm wear weak in astro vans replaced mine2 in 5 years

Assuming...you get the noise with the vehicle moving; try loading up one side at a time by doing lane changes when you hear the grind. If the noise increases & then decreases as you go from side to side then you probably have a bad wheel bearing. Also, You could just have a pebble caught between the backing plate & rotor! (they have a tendency to fall out quickly)