Truck won't backup when loaded

[scott75k20]

Need help 75 Chevy K20 4x4 standard transmission - when I have the truck moderately loaded I can not back up, when I let the clutch out the truck bucks violently. If I put it in low the truck will back up. This has been an on going problem 3 years ago the problem got so bad that it would no longer back up in low, replaced clutch and pressure plate which then made it possible to back up in low gear. Don't have a problem when the truck is not loaded.

[SomeTexan]

Does the shifter try to pop out of reverse? It could be rounded shift lugs on the reverse gear, or a bent shift fork not letting it all the way into gear.

[blazing816]

does it go through the forward gears when you have a load???

[scott75k20]

No the shifter does not pop out of gear and I haven't had any problem going through the forward gears.

[roundhouse]

Check the mounts.   For the engine and for the trans/t-case.  To the crossmember and to the frame
Also check the clutch linkage closely

You saying the clutch just slips?   
Or you just have a whole lot of clutch chatter?

You mention if you put it in low it backs up?
I guessing you mean 4-lo in the transfer case?

Could be some problem with the bearings in the trans or output shaft but  I dunno

Does it act differently if you are backing up a hill loaded?
Up a hill empty?
Down a hill loaded?
Downhill empty?

Or just of there is weight in the bed  no matter what?

If its just when you have weight in the bed. It could indicate some problem with the slip joint in the driveshaft maybe  binding when the back of the truck is compressed

Have someone jump up and down on the back bumper while you watch the slip joint
Make sure it's sliding freely

Check the bearings where the driveshaft goes into the rear axle too

Chock the wheels  put trans in N and start inspecting everything with a pry bar and a very bright flashlight like a sure fire or a fenix

Have someone else try to back up the truck while you look at the trans and tcase mounts
Use a bright light and stay back so you won't get run over if the truck moves

While your helper is attempting to back up watch the rear axle.  And the pads where the axle mounts to the springs
They can get worn into a bananna shape allowing the axle to rotate 
If it rotates enough it can bind the Ujoint


Place one of those halogen work lights under the truck so you can see

If you have a go pro or some video camera.  Mount that under the truck at various places.  Aimed at the clutch linkage.  Aimed at the t case cross member etc and give t a test drive.   
Sometimes thats the only way to spot some odd problem or movement
Do you have the slip yoke in the t case ?
Or built into the driveshaft ?

[VileZambonie]

Check the mounts

X2

[scott75k20]

The clutch does not chatter it seems to engage momentarily then lets go violently. The truck bucks so violently I'm afraid that something will break. Yes I am putting the transfer case in low gear. When trying to back up a hill I have the same problem maybe more so. The truck is ok going forward and does not feel any differently loaded verse non loaded.

[bake74]

     If your mounts are good as  roundhouse & VileZambonie suggested, then it seems to me to be an internal transmission problem with the reverse gear (in transmission, the actual gear set).  The teeth might be worn or partially broken, so when you put a extra load on the gear set (ie loaded truck) it "lets go" as you said. 
     The other possibility could be the reverse gear set tolerances is not close enough anymore, thus the added stress will also make it pop off the gear set.
     When you put it in 4 low, the front axle is helping to push, instead of the rear axle doing all the pulling.  That is probably why the transmission is not popping out in reverse in 4wd.  Going uphill will compound the problem since more of a stress is put on the gear set.
     I would suggest it is time for a transmission rebuild and paying special attention to the reverse gear set when you have it taken apart.

[roundhouse]

If you have the removeable cover plate in the floor you could take that off and remove the top of the trans
To take a peek inside

If you do that remember to put the shifter halfway into reverse before you remove the top cover of the trans

I'd still say the most likely problem is a broken or loose mount allowing the trans to shift slightly which causes  the clutch linkage to engage or release the clutch

Just like if you were moving the pedal up and down with your foot