poppin out?

[jewels.]

my little truck was havin some problems lately.... she's POPPIN out.....

the first time, it only happened once the entire trip - about 250 miles of driving....

but then the last time i drove about the same distance, and it did it about six or seven times....

i get up to a good highway speed of 70-75 mph....
there is usually a strong vibration at that speed - not a bad vibration - just a typical driving vibration....

and then all of a sudden, the vibration will stop.... the truck will drive EXTREMELY smooth, and then a minute later - POP - the transmission will kick the shifter out of 3rd gear into neutral.... (its a 3-on-tree)

its definitely a significant shove from the tranny - almost a 'chunk' - causing the shifter to go into neutral.... i can feel it in the shifter when it does it.... (cause i kept my hand on it, when i would try and duplicate the problem)

but it will only POP out of gear when i do 70-75, and only once the vibration goes away (suddenly, but smoothly)....

it will NOT do this if i drive 65mph, because at that speed, the 'vibration' never dissappears.... (strange)

so - is this a clutch problem? (i have never replaced my clutch)....
is this a tranny problem? is something worn in my 3-sp-on tree? its a very strange thing....

on a side note, the first time it happened (when it POPPED), it scared the **** out of me because i felt the chunk from the tranny, and thought for sure something had broken, and i would be stuck on the side of the road.... but then i discovered it had just come out of gear.... (which made me breathe a sigh of relief! whew!)

suggestions? comments? solutions?
thanks!
jewels.