It will only drive in 3 and will not go in revverse

I means the transmission pumps pressure regulater is failing
that VAC line tells the box engine load. and if bad can cause
transmission to shift wrong. upshifts early or late.
but none the less it is bad fix what you see is bad.
does this box have TV cable, from TB TV valve top transmisssion
if yes it too must never be jammed, it is, transmission damage deeper happens./
they both control pump output main control pressure.
VAC modulatori and the TV cable. (throttle valve = TV) from TB throttle body
it is pump pressure issue on the main line(inside)

yes 800 rpm is slower than 1500 cold RPM an the pump runs slower so yes, pump output is less.
the transmission manual has page covering pUMP output port tests.
in all gears to see it fail. all transmission shops this is one of the first tests. besides, cable TV bad or VAC mod bad.

With the van in park or neutral with e-brake on, and engine running, pull the transmission fluid dipstick, notice there's no fluid on the dipstick. Put a quart of transmission fluid in the dipstick tube, with e-brake still on, put the van in gear. R or D, and see does the van try to move against the brake. If it tries to move, engine speed drops, put it back in neutral or park and check the fluid level again. If it's still low, put in a little more till it's to the full mark.. Done. Check under the van and find the area where fluid is dripping out and fix it. If it's the pan gasket, replace it. Replace the filter while the pan is down.

Needs a rebuild or replace with a refurb or go to junk yard. I would say replace the bands but it sounds like it's pretty toasted

Probably too late but for other users searching: You probably have the 4T65 transmission like I did. This is the vehicle "hunting" in and out of overdrive on the highway (marked "D" on your dash) and you're probably on level (or close to) ground. You have two options: drive in the gear selection "3" (more fuel consumption though) or change your speed. I used to drop down to 53 MPH and it would stop, or on the highway @ 65 for it to go away.
Seems to be common in this trans around 60 MPH. Also - start saving for another vehicle or repair. This trans will start to slam into 2nd gear from a stop after it warms up (internal solenoid fix required) OR it will shudder horribly from a stop after a highway run. My fix was to replace the solenoid in the trans (there's video on YT) by pulling the right side of the drivetrain off the van and pulling the trans cover. If you go this route - you *must* ensure no -and I mean NO- dirt gets in that tranny while the cover is off.
Good luck!

u have probaly lost reverse but u shold be able to drive forward unless u have lost every gear and not just reverse call taxi r rent a car are buy anew r used car not being funny but a transmission 4 that u could find u a newer ride

This sounds like you may have broken a transmission line or damaged the transmission pan. the oil has leaked out of transmission that is why it will not move,

Do not start car until you have repaired the leak, running the engine will damage the transmission due to low oil in it,

you will need to add 2qts of transmission fluid then jack up the front of the car and look for the leak, if you do not see it then have someone start the car and look, you will be able to see it leak out, i thing it will end up being the pan, good day.

i have had this problem before. driving around 45mph still in second gear. when u let off of gas van lurches as if you were downshifting to stop. however, van is automatic. doesn't make sense. van is low on oil and causes gears to stick. like some insane nightmare. give it a quart of oil and you will find the van easily changing gears and acting normal again. don't ask me why or how, but i speak from experience

sounds like transmission

Sounds like a dead transmission.

i have the same problem when iam driveing it kick hard into 1 gear and 2,,, shut it off,,and it stops for about a day or 2,,, then comes back,,, it is still under warranty,, and mr,trann said the montanas or bad for valve bodys,,, i,ll let you know how i make out,,, but my shop said he was going to rebuild the hole thing,,,witch is good beecause they put all up grade parts in,,,,,, unlike the **** GM parts