Transmission slips only in first gear
It is started on a raod trip from California after going through higher elevation passes. 1st gear slipping from a stop, especially when temperatures got over 220. Slipping continued worse after warming up the whole trip home, down to around 600ft above sea level. After being home a few days, it barely does it anymore, even over 220 degrees and and hour of driving. I thought I'd need a new tranny but now I'm concerned it's something giving me the wrong signals from being as high as 9000 ft. Also I have no idiot lights or codes coming on and the fluid hasn't darkened or smelling bad. It's 1700 for a new tranny and I can't tell anymore if that's the cause. Also it has no emblems other than gmc 1500, registration says only k1500 and that's not an option looking for for parts. Even here k series doesn't give 94 as an option and all the title says is k1500.
For a 1994 truck and the symptoms you have, I would bet the transmission is slowly wearing out. I'd have a transmission mechanic diagnose the truck if you are concerned, or just drive it till it won't shift at all.