When you say "top gear" I think you might be feeling the "torque converter lock-up" which our trucks have. If you turn off the radio and pay attention when you're driving your truck, you will feel 1-2, 2-3, 3-4 shifts and then you will also feel what is like another gear shift. It's the torque converter "lock-up" which is designed to remove any slippage in the torque converter. It's designed to lockup at about 50 MPH an disengage at about 45 MPH. It also engages and disengages when under load or accelerating. A good scan tool that provides live data and provides transmission codes, will be able to let you know if there's a problem. It works off fluid pressure and I think is the earliest indication of low fluid level. Since you just bought the truck and probably have no idea what the past service records are, I'd change the transmission fluid and see if that helps at all.
As far as the coolant goes, the previous owner may have replaced a hose, water pump or thermostat and just grabbed whatever coolant he had and put in there. The only problem I see with using either of these coolants is that they don't "play well" together. If you have any idea that you might have the two mixed together, I'd drain, flush and refill with whichever you like the most.
When I purchased my truck at about 180,000 miles, I assumed that no real service had ever taken place an I changed the oil, transmission fluid, rear differential, front differential an transfer case fluid, replaced fuel filter, air filter and spark plugs. That was a pretty expensive trip to the parts store that day, but I wanted to make sure I was giving this use truck the best chance I could to be dependable and trouble-free. It also gave me a great opportunity to become familiar with the truck and where everything was and how it was all put together.
Change that transmission fluid and see how that works. In the "how-to" section here you'll find tons of information on how to do all the things I've suggested if you're not mechanically inclined. Also, there's very little that you'll experience with your truck that we have not already "been there" and found out how to fix it, so let us help when you need it. You've found a very valuable "support team" here an we like the help!
Welcome!!