Have an 02 Suburban 4x4 1500 at about 85,000 orig miles. Truck was fine until sometime last week when all of a sudden, I could feel the truck didn't want to shift from first to 2nd until i released my foot off the gas pedal and then it would shift or I just wait until my rpm is high enough and then it will shift. I am not sure if this is a 4 speed but lets just say it is. I can't seem to get to 4th gear. Running fine but my rpm at 3rd is high and won't upshift to 4th on the freeway. I thought maybe I had the tow/haul stuck but no light indicator that says otherwise.

Is there some kind of way to determine if it is electrical or hydraulic? Could it be a stuck solenoid or electrical switch? I checked the tranny oil and it was fine with levels. I was actually planning to get the tranny oil/filter change but this happened before I could get it serviced but now I am rethinking if I should even or just have my tranny rebuilt? Any suggestions or clues before I take to the tranny shop?