sounds like tranny slipping out of OD. While in OD and going up grades you get a "bucking" and if you give it more gas it will downshift and no more bucking. My 96 does the same thing and as I have seen it is very, very common. As far as fixing it, could not tell you. Different people have found different things to work, but could be failing trans. For me, I have been upgrading my rig and recently added 31" mudders which effects my shifting and speedo, and as a result I have not noticed the problem since. However it is important to note that the reason for not having the issue is that I am likely not in OD going up hills anyway. Nevertheless, search for "bucking" and see what you can come up with.
) when going up grades, over passes and such. changed air filter and fuel filter and still does it.