At a loss
 

On my 86, i've put in 2 700R4's and i keep having the same problems. I lise all gears when i apply brakes hard. I have to turn truck off in neutral so i can get it into park without grinding. I ruined first tranny. This one now is doing it occasionally. What's causing this?

If you are stopped and transmission grinds while trying to put it into park the trans is still turning with nothing behind it to hold it - If you have a limited slip or posi differential, My guess is clutch packs in the differential are getting weak and slipping - same thing would happen on a maximum hard braking trans will rotate and differential will slip

Ummmm, I don't even come close to agreeing with the above post.

Think torque converter. It's not what is behind the tranny that is supposed to keep it from spinning, it is what is between the tranny and the engine.

Torque converter will provide the same as a clutch being depressed on a standard trans, no power too the transmission, if the vehicle is at a full stop and grinding into park the transmission is still spinning. The park pawl is at the rear of the transmission behind all clutch packs, Yes with a torque converter there should be no power thu the transmission but it also would not grind when putting it into park, if the lock up clutches in the torque converter were staying engaged it would stall the engine. So the problem should be behind the transmission. AJBERT - Unhook your driveshaft and lay the differential end on the ground, Start the engine and then put the vehicle in gear or even in neutral, the drive shaft will spin and it will beat on the ground without the differential and brakes holding it from spinning. And when trying to put the transmission into park it will grind the park pawl until the transmission stops rotating. Upon a hard braking and such once the clutch plates in the differential slip its a freewheel. The weight of the vehicle along with the rear brakes and differential is what keep everything from turning not the torque converter.

The truck doesn't have posi rear and each time i've replaced the tranny, it's been a different TC

A limited slip differential has a clutch pack it it also same as posi

It only has straight 456 gears

Well at a full stop if the park pawl is grinding the transmission is still spinning so something behind it is freewheeling - I'll have to think on this one some more - maybe someone else will chime in too

Thank you. Any help is appreciated

Ok lets skip the grinding into park issue for now - when the new transmissions were installed was the TV cable set to the proper settings?