When the truck is on jack stands on the rear I can put it in gear and the brakes won't stop the rear wheels unless I put it in neutral but they feel good when its rolling on the ground what could it be

If your saying that with the truck idling in gear, the rear brakes won't stop wheel rotation, the rear brakes are not working properly.

There is not an anti-lock feature for the rear brakes is there? Rear end in the air, will reduce or stop rear brake function if the anti-lock system is functioning.

If no anti-lock system is present, I would begin with checking the adjustment of the shoes, they may be too loosely adjusted. Then make sure the shoes are not glazed over or grease or brake fluid soaked. A lot of brake dust will also reduce the effectiveness of the brakes.

Beyond that, the concern would be not enough brake fluid movement or volume to over come the return springs and apply pressure of the brake shoes against the drum. Look for leaks, bypassing master cylinder, or a not centered brake proportioning valve.

Then be sure the pads the shoes rest against on the backing plates are smooth and lubricated so the shoes can actually move and are not stuck in groves or stuck to the pads.

Rear drums that are worn too thin can deflect when brake pressure is applied and can make the brakes less effective.
Gene

Everything checks out they are brand new shoes, drums are used but not that thin,and new brake hardware, springs, etc but hoping its not the proportion valve I got another one if it is the cause brake fluid is full, no ABS on 84 trucks

The 3 raised pads on each side of the backing plate that the brake shoes sits up against are flat and smooth, right? I've seen a lot of those raised pads have grooves wore in them, and then the shoes can't move in and out like they need to move. Its something easily over looked, and it doesn't take much of a grove (on any 1 of the 6 pads) to interfere with the brake shoe movement. Gene

Are the shoes adjusted to where they have a slight rubbing sound when the drums are rotated? Are you getting good pressure out of the bleeders?

That's my next step is bleed the rears and adjust the shoes again

My 1980 was doing that last summer and it ended up being both brake cylinder had seized up

spoon the shoes up tight while wheel is spinning until it stops 1 tire then back off till it barley drags/hear it.

then do other side same way.

open diff = 1 wheel off ground at a time.

sg or locker = both wheels off ground.

remove and clean the adjuster and lube it. push the wheel cyl pistons in to make sure they are not stuck/frozen up. move shoes around on backing plates and lube backing plate/shoe rubbing points.

** make sure adjusters are on correct way/side just in case someone else goofed.