Truck: 97 4.0 ranger, 5 speed, 2wd, 4:10 gears w/33" tires

This problem just started after I replaced my transmissionwith a rebuilt 5 speed, not sure if it could be related. My truck runs excellent up till about 75mph then it will hesitate for a fraction of a second, almost like hitting a governor. Now the tricky part is that it does not do it everytime I hit a 75mph, sometimes it will let me go to 80mph before it does it. When I hit 75mph in 5th I am at about 2600 rpm and in 4th I am at about 3200 rpm. It will cut out in either 4th or 5th and I can only guess it would do it in 3rd. It is not consistant in its frequency (may happen once/minute sometimes five times....No CEL and no codes stored.

Any chance it could be the speed sensor in the diff?

Any words of wisdom? Thanks for the help!

Scott