A major upgrade I've put off is replacing the stock TH350 trans with a 2004R. Two weeks ago I had the overdrive trans installed. My '79 vette drives well with the new trans...almost. On 9/2/06, while on the freeway, I heard a "pop" and then instantaneously no forward gear response. I could only coast to the side of the road, and wait for a
flatbed to haul me home. I've been in a state of suspended belief. How could a new trans fail...especially one built by a famous company? I've checked the trans fluid..OK
I can put the gear selector in park, and it holds. I started the engine, put the car in reverse...the sideshafts were not turning. I can't see the driveshaft as the car is low to the ground.

Would the problem most likely be the differential? Less likely the torque converter?

If it is the differential, is the best policy to use GM Performance Parts, or go aftermarket...such as Eaton Posi Limited Slip Differential?

Appreciate any comments from anyone who understands this situation.

If you have a way to lift it you can take the back tires off the ground and put it in gear, (be sure to keep you foot on the brakes, not the best on the tranny if you don't) If the drive shaft turns and neither one of the tires turn it is your diff if you drive shaft turns it is your tranny.
How long has it been sense the diff fluid has been changed? It may be worth taking the cover off and looking at it.

As long as you don't have a silicone gasket on the diff cover, you can use synthetic. Synthetic gear lube would eat away at it. I would recomend checking the spider gears too. They tend to have pits in the gears with age. That's what happens when you don't change your diff fluid, metal shavigns accumulate and the acidity goes up.

This is an old thread...I assume you've got the problem taken care of by now, but...
To tell if it is the trans, or diff, watch your speedo...if the speedo registers, but the car doesn't move, the diff is broke.

I'd bet it was the converter coming out of the pump gears, and breaking them...[:@]