'99 explorer transmission bucks

[blindsquirrel]

the trans on my'99 explorer bucks, when going up hill especially and more when hot. the truck is at 135k but i got it at 128k so i don't know what was replaced in the past. it seems to stop when switched to 4wd. any ideas?

 

[CypherSTL]

I had an issue "similar" to that, with the bucking then stops when put into 4x4. Replaced my hub bearings and that was solved.