Transmission or Rearend Problem?

[Sixball]

I put a 454 and TH400 in my 94 Shortbed Standard Cab Chevy 1/2 Ton. Total computer delete. I also installed a Power Trax “lunchbox locker”. No issues for about 200 miles. Tuning the 454 and doing a few burnouts with no issues until last week. 3rd Gear take off from a dead stop, laid rubber for about 30 feet then the engine just revved while still moving forward. I let off the gas thinking I broke something but Truck continued moving forward. With my foot still off the gas, I hear the back tires catch like power was going to them, I slightly press the gas and it drove fine for the remainder of the trip ( 10 miles).

I checked the U Joint, no slop, tight bolts etc.. I drove another day about 25 miles, no hard acceleration. No issues until almost home and I felt a clunk - with the rear tires catching traction again while just driving normal at 30 MPH.

Where do I need to start looking? Rear end or Transmission?