96 blazer. wont shift

I have a 96 blazer with a 4.3l v6 vortec and I took about an 80 mile trip ice fishing and when i came off the ice 6 hours later i started off driving down the highway and my truck wouldnt shift into 4th gear. i couldnt drive over 45 mph and then i stopped at a gas station and when i left i could get out of 2nd gear. the rpms would just spike and it wouldnt shift up. any help would be greatly appreciated... thank you

How is the fluid level.
Any check engine or transmission lights on?

May be time to get it on a good transmission/engine capable scanner.
Post back any codes.

Yes the fluid level is full. I had to leave it at a gas station because it was pretty far away. He said my tranny was junk and he started it up to move it and it didnt even want to move at all. do you know of any places that I could buy a rebuilt tranny at a fairly good price? or of any place where i could get my tranny rebuilt? I might want to add that the tranny has 204k on it. The guy at the garage said it was "toast" i did notice the tranny was smelling a few days before but i didnt notice any slipping or anything until the day it happened it would shoot up in rpms before shifting...my stupid self didnt think that this would happen of course. also the only codes that i have had are system running lean bank 1 & bank 2. once again any help would be greatly appreciated.

I would check out the old one good.
Drop pan and check filter and see what is in the bottom of pan.

Call some bone yards and see what they have and ask mileage and warranty.

A lot of places rebuilt them.
But check them out on warranty and prices.
Stay away from the big transmission chain stores.

Jasper and gm factory have good quailty rebuilt transmissions with good warrany up to 3 year or 100K warranty but also a high end price.

Good Luck