really? 300 hours on a truck made over 10 years [and maybe as much as 20 years] ago?
Do you believe it? Because that few hours over that long of a time would cause it's own set of problems.
As far as "harsh engagement of the transmission" the only PSBs for this that I am a ware of, concerned both directions, not only in 1 direction (same parts# used in both directions).
?Have you checked to make sure all the motor mount bolts are tight, and none broken (put a wrench on all the motor mount bracket bolts)?
have you taken pressure readings of the trans?
I am betting you have a broken or damaged clutch disk or clutch housing worn out, or sticking clutch plates (if it really only has 300 hours after 10+ years, then sticking, if worn, then you have lots more than 300 hours...