Blazler "kicks/jolts" when shifting from 1st to 2nd gear

Hey guys, I really hope someone can help me out with this problem. I have a 2000 Chevy Blazer LT 4WD 4.3L. I came home from out of town today going 65-70mph the whole way back. I got to a stoplight back here in town and starting going and noticed a slight "kick/jolt". After a few times of it happening I determined that it was when the car would shift from 1st to 2nd gear. That's the only time it did it, shifting after that was fine. I checked the tranny fluid and it looked fine to me as well as the other fluids. If you're still confused, think about when someone makes a bad shift with a manual transmission and that's essentially what it felt like. Please, any info is appreciated.

Is the check engine light on?
Are there any codes in memory?
How many miles since the transmission fluid and filter were changed?
How many miles on the transmission?

Could be a sticking or binding valve in the valve body.
Could be a sticking 1-2 shift solenoid.
Could be a poor connection at the 1-2 shift solenoid.

Is the check engine light on?
Are there any codes in memory?
How many miles since the transmission fluid and filter were changed?
How many miles on the transmission?

Could be a sticking or binding valve in the valve body.
Could be a sticking 1-2 shift solenoid.
Could be a poor connection at the 1-2 shift solenoid.

No check engine.
Not sure on codes.
Not sure when they were changed, I've had it for 3 yrs and 50k miles without changing it.
118k on the transmission.

It's not doing it now for some reason.

There was a TSB a few years ago, about that very same thing happening intermittently when hot on the 4L60E trans. It had to do with a valve binding in the bore of the valve body. GM has since fixed the problem, but the possibility still exists for the valve to bind if there is a build up of dirt on the valve itself, or in the bore. The tolerance is very close, so it doesn't take much.

50K is a bit much, and who knows how long it went before that! It's over due! Depending on how it's driven, (city, highway, pulling a trailer, etc) the transmission should be serviced every 36K. Changing the transmission fluid and filter takes about an hour. The filter and gasket kit is usually under $20 and it should take almost 5 quarts of DexronIII fluid. Use a torque wrench when tightening the pan bolts to prevent leaks.

I had the same problem with my 97 when it got hot it would shift hard from 1st to 2nd, Replaced the valve body and the problem went away. This was a few years ago and I think the valve body was around 400 bucks.