Identifying transmission problems

I havent had very many automatics in my lifetime, mostly manual transmissions. I recently bought my 2000 Hoe with 181k on it. Today while driving home from work I noticed when the tranny shifts into 2nd or 3rd(not sure which), the rpm's raise about an extra 1000 rpm in between these shift points. It only does this under 75% or more throttle. Is this a sign I need to start looking for another transmission? Fluid level was about 2 inches above the full hot level when I got home, and a little dirty. I put it in 4x4 low and climbed a fairly small but steep hill slowly and it seemed the power to the wheels would surge, but it pulled the hill without much effort other than that.

Side note, I just put a set of much heavier wheels and tires on it, 285/65/18 Nitto Grapplers.

Your tranny oil should always look clean and translucent reddish. Any darkness, brownish color is bad. If your smeller works good, take a sniff at the trans dipstick, burn smell is bad. The shift delay could be caused by sticky valves, worn/slick clutch packs. Aamco does a free diagnostic, could check that. Or could have the tranny fluid changed, inspect the old fluid and bottom of the pan, ask the oil-change techs what they think...