a way to check the u-joints is to putthe -brakeon andthe ruck innutral and get under the truck an turn the drivshaft back and forth and look at the slop in it, if there is no slop,it could the could be the solenoid

 

the part may be pricey. but the labor shouldnt be hard. you'll have to obviously drop the pan and the solenoid should be at the bottom of the valve body. good luck

 

does the truck have a quick shift kit?
I have a quick shift kit and when warmed up mine shifts hard from 1st to 2nd when I am at or over 20% open throttle.
and as said check the tranny fluid. Also could be shifting solenoid.

 

Thread search this, apparently there is a plastic ring in the 4L60 trannies which fails.

As I recall, we hashed this out pretty thoroughly about a year back...