we have a 1985 e150 straight 6, shifter on floor(3 speed manual with overdrive), engine starts and works, van won't drive forward or backward, disconnected front brakes(thought they might be locked), no difference, jacked up rear driver side wheel then rear passenger rear wheel(not both at same time), put van in gear, rear tire spins forward in drive and backward in reverse, same with other rear wheel, take van off jack stands, van won't drive forward or backward

 

Sounds like a clutch problem.

When you depress the clutch pedal , there should be about 1-2 inches of"free ply" before you feel any resistance.
If there is no free ply, clutch might need adjustment to obtain that, meaning clutch is lot fully applying.
If there is adequate free ply, clutch disc is worn and will require replacement..

 

thank you for the reply
we thought it might be the clutch, but why do things work(clutch, gears shift, tires turn in gear and reverse)with the rear tires on a jack, but with the rear tires on the ground, everything works, but the tires don't turn in gear or reverse

 

With the wheels off the ground there is little resistance to move them., but when they are on the ground the whole vehicle is trying to be moved and because the clutch disc is worn OR ASSEMBLY NEEDS ADJUSTMENT, the clutch is slipping..

Do you have the free ply I suggested? Yes/No

 

we won't be with the van for a few days, we'll let you know about the free play as soon as we know, you said to adjust the clutch, the van has a hydraulic clutch(main and slave), how would we adjust the clutch

 

The slave cyl still mechanically moves the clutch fork and there is a threaded rod there , the length of which can be altered..

Before changing anything, is the slave cyl responding to movement of the pedal in the vehicle? It should respond according to the movement of that pedal inside the vehicle.

 

there's free play in the clutch pedal, seems to be working good, the slave cylinder rod works good when pressing the clutch pedal

 

It appears the clutch assy may be in need of replacement.
Because there is a good amount of work and expense involved, I would suggest you consider having a good shop verify what you have checked and you can plan what you want to do after that.
What is usually replaced are the pressure plate, clutch disc, pilot bearing on the flywheel, and maybe the slave cyl if it shows any sign of leakage.
Because of the amount of work involved,we don't want to go back a short time later to replace a small part.
This had to to be showing signs of slipping before it got this bad..