Th400 slips above 50 mph

[DD38]

I have an 81 jeep CJ7 with a sbc 350, th400, and bw1339 transfer case. I had the transmission rebuilt about a year ago. I put around 3,500 miles on it and one cold morning it started slipping. It would slip only when traveling above 50 mph. I would let off the gas and it would catch and drive fine. It did this 4 or 5 times in a 35 mile trip. On the return trip it started slipping to the point that I had to drive below 20 mph to limp it to a safe place to park. I pulled the transmission and took it to a shop for a rebuild. The invoice shows that a master kit with friction, steel module, shift kit, modulator, solenoid, filter, and torque converter were installed.
After installing the newly rebuilt th400 and filling it with fluid, I ran the jeep on jack stands. Everything seemed good. It backed out of the garage fine and turned around in the driveway. When I took it for a test drive, I slowly accelerated and everything was good until I got to approximately 50 mph and the engine suddenly revved high. I let off the gas and made a u turn. The vehicle again accelerated fine but I only went up to about 35 mph to avoid any possible slipping. Back in the garage and on jack stands it seems to work fine.

Pressure test while on jack stands with vacuum modulator unhooked
P: 165
R: 260
N: 165
D: 160
2: 180
1: 180

Pressure test with vacuum modulator connected
P: 100
R: 160
N: 100
D: 95
2: 180
1: 180

The modulator valve was also checked. It is not leaking and moved when vacuum was applied. The fluid is clean and does not smell burnt. I used Valvoline Dexron VI as instructed by the builder.

Sorry for the long post. I wanted to provide as much information as I could. Any ideas, advice, or suggestions to assist with trouble shooting this would be greatly appreciated. The transmission is currently installed in the vehicle.