I think this is a transmission problem, just assuming the worst. here are my specs: 79 c15, LT1 swap, rebuilt th400, auto/manual street strip valvebody, dual feed mod, jegs antiballoon 4k stall. rear end is a ford 9 in built 35 spline, spool, etc... put 4.10s in awhile back.. it was my last day home from college when this happened so i didnt get to check for shavings on drain plug or if the diff was low or anything like that, trying to figure out what i should check first when i get back in a week.. truck is loud as he!! so for me to hear it part throttle it must be bad. really hoping its just the ring and pinion but i dont know if these are symptoms.

The other day i was out cruising about and went down a huge hill, in efforts to save my brakes, i came to a complete stop and put the tranny into manual first, i then let it engine brake down the hill and at almost the bottom the motor quit and i coasted to the bottom after bumping it in to neutral. at the bottom i stopped started back up and went back to cruising. later on i went back out on a two lane (after holding it on the floor a couple times trying to figure out if something was wrong) then when applying light throttle around 50mph the truck makes grinding noises and makes a whining sound that varies in volume. however the couple times i flat footed it about 50 the noise was initial and i couldnt really hear anything after moving ( once again, truck is really loud so i may have just not been able to hear.) the grind could also be heard when you first let off the gas. it wont do it in reverse or park. if cruise at 50 with foot just laying on the pedal the whine got really loud.

Any thoughts would be greatly appreciated.

 

Sounds like you might have a drive-line noise... but it is very difficult to try and figure-out anything really from the small description provided.

Thank you for providing info on the vehicle type and Power-Train... Two-Wheel drive with Leaf-spring rear suspension?

Gen 1 Small-Block Chevrolet engine, 350 with LT-1 cylinder-heads and valve-train, THM400, and a spooled Ford 9" upgraded with 35-spline axles.
Hopefully that was all correct? What are you using for a drive-shaft? Any aftermarket improvements to the rear suspension?

Street driving with a spool in the rear differential can definitely accelerate wear-and-tear in the Diff, drive-shaft, and Trans.
You might even be snapping the rear axles if they were smaller diameter, lower-spline, weaker material, Etc...
Uneven tire wear from being dragged around turns, must not be too fun.

Noises can be especially troublesome to locate without inspecting the drive-line components...
Driving the truck at safe-speeds on an appropriate lift; following as much safety precautions as possible and listening under the vehicle...
This is where I would start.

Then remove the transmission oil-pan and then pull the center-section of the rear differential (if the transmission looks clean inside).

We can proceed with some more in depth inspection from there if necessary.