The key to any GM 4sp Overdrive transmission is to keep the torque converter locked up in 4th gear. If your torque converter unlocks in 4th gear and doesn't downshift to 3rd gear right away, that is a bad bad bad thing. If you can't tell when the torque converter locks and unlocks, then manually pull the gear selector into D (3rd gear) and leave it there.

Since he lives in MN, I'll assume his truck is 4wd. Nearly all Trailblazer 4wds have at the 3.73 gears, with 4.10 being an option.
If you really want to know, TEG, look in the glovebox, and the sticker in there will have a bunch of three digit codes. Go to the "G"s. GU6 is 3.42, GT4 is 3.73 and GT5 is 4.10.

I could be WRONG....but i'm about 99.999% sure it has tow-haul.

If it has "3" as a gear selection, then it has T/H. Otherwise O/D off would be "3". The 4L60 (which is what I think it has) i think came standard with T/H. T/H will keep the T/C locked up on the highway at least. However, honestly, your safe bet is to keep it in 3rd. I always use 3rd with our Tahoe because it hunts too much in O/D with T/H on. We're only talking a 4000lbs boat.

PS: Late night, extremely tired rambling alert.

3rd gear only when towing for me. And virtually no cruise. My 4.6L runs around 2,800rpm @ 65mph while towing on ~flat terrain.

When my wife gets home I'll investigate.

The one time I've been able to take my boat out this year, I pretty much left it in 3rd. When I was goin on level or slightly down hill I put it back in OD. Anything goin up or downhill back in 3rd I went.

Needless to say I was in 3rd most of the time. Seemed as tho thats where my truck liked to be..And, yes my truck talks to me..When it does I listen...

Third Gear and NO OD for me -- I tow with a 4.6L Expedition. If put it in OD it takes more throttle are returns LOWER gas mileage than in third --at the lower RPM of OD the engine just doesn't develope the needed torque. I am also at max weight limit.

On long flat highways, I set the cruise controll at or near highway maximum speed limit (generally within 5mph) and drive, in OD (4-speed automatic).

In hilly terrain ( I mean really genuinely hilly), I'll go to 3rd (like in southern Mo/Northern Ark, last weekend). A slight rise that kicks it to passing gear does not get me out of 4th.

The manufacturer and manuals all say to tow in 3rd. If you blow out your tranny will your warranty cover it if you tow in O/D?. For a few cents I always tow in 3rd.

I just use the tow/haul button. It still allows it to be in 6th, but the shift points are changed and I rarely get going fast enough to get it in 6th.

I'm similar to Dale. We have an Expy with the 4.6. Unfortunately, I also have 3.31 gears. I can pull on flat land with OD, but ANY rise in the highway forces a downshift to maintain speed. I generally go to the lake with OD off (generally uphill for an hour) and turn it on for parts of the return trip. With OD off, I can cruise at 65 turning about 2400 with power to spare. If I remember right. OD on drops me to 1600 and no power. AC makes a noticible difference towing in OD. I added a pretty oversized tranny cooler to help handle the heat.

I use cruise if the ground is reasonably flat. Turn it off in the hills though. If I try to maintain 65 up a couple of the hills on my lake route, I would be in 2nd turning over 4k RPM's. I just move over to the slow lane and cruise up at 50-55. I think at 4000 RPM, I can literally watch the gas hand moving....

The 3.31's are beautiful without the boat though. Can set the cruise at 58 and make the trip to my hometown, ~200 mile round trip on 2 lane roads, averaging 21-22mpg. Towing down the interstate gets me 10-12 there and 12-14 back. 4th of July, I got 15 back.