2000 Isuzu Trooper transmission range sensor

My car was starting to show symptoms of a transmission problem about 2 1/2 months ago. When nearing between 40-50 mph it had delayed shifting and would then jump forward. It also had a delay when shifting into reverse occasionally and would jump backwards. My last trans fluid flush and fill was in March of 2007. Since then my vehicle has driven back and forth to Texas twice and to Colorado and up a mountain. I called the dealership to have the fluid checked and they felt that instead I should fix the transmission range sensor. The fluid was never even checked. Would the transmission range sensor be causing my shifting problems, or at least is that a possibility?