Your very lucky you were not more seriously injured!...I've seen 2 guys crushed under cars they were fixing in their yard..one was using wire milk crates for jack stands--he's dead!..so is the other guy who trusted his OEM bumper jack to hold up the car while he was changing his tranny fluid.one wheel was removed so he could get under the car better..too bad he forgot to slide the tire and rim under the car--he was crushed to death,and his face and chest had the outline of a TH350 embossed into him...he wont have to worry about his tranny anymore!..
You should use something in place of the floor jack's "saddle"--weld a peice of pipe to a flat plate,and wrap a chain around the tranny and T-case and bolt it securely to the flat plate ,so it cant fall off and crush you...a NP203 or NP205 T-case is definately not something you want falling on you!--they weigh at least 250 lbs,attached to an SM465 tranny,they are over 500 lbs!..A helper is an excellent idea!
..Even having your 5 year old nearby is better than no one..I've done several alone,and always feared I'd get pinned under the truck and noboby would find me till it was too late!--but I used a tranny jack adapter with my floor jack,chained everything in place,and worked carefully--so far I'm not dead--YET!...but all it takes is one moment of haste to END ALL your moments forever!--BE CAREFUL!...