I grease with each oil change also. But haven't had to worry about 'over-greasing' the slip joint. My driveshafts will move a bit at a certain point of 'fill' but the seals around mine let any extra seep out when under compression. IF your seals do not...then it could be an issue.
I have over 300K on my rig (original U-Joints and driveshafts) and don't see any indication that I have been pounding the bearings in my T-case or Diffs.
Not saying it isn't possible...but I doubt it happens very often. SNIP
Actually, this happened the very first time I serviced my still nearly new FJ55 back in the day. I'd previously owned a '76 Subaru 4x4 Wagon, which IIRC even then mostly had sealed bearings, but which I may not have owned long enough to jack up to grease the driveshaft on (which may have had the only zerk on the thing.) Pumped that baby full of grease - if a little lube is good, more is probably better. Finished the service, headed to work and, oh, the bad vibes!
Fortunately, I had a pretty good mechanic brain trust around the break room table at work. IIRC, we pretty much figured it out, but I had to wait until I got up the next day to pull the zerk, which spewed grease. Problem resolved. Excellent, I hadn't trashed my new Land Cruiser! Because it sure seemed like it at the time.
Yeah, it might leak out, but could you really stand the irritation? Not if it was as full of it as mine was, which was hammering and surely would've done seals and bearings near either end of the DS no good.
I pump the gun until I see the very first motion of the slip yoke, then it's done. That's worked for me ever since that first tomfoolery.