Check for leaks in all the gaskets and seals. Chances are, if a seal is leaking, the bearing behind it is shot. Ask the owner what kind of lubricant they're using. Synthetic gear oil will often manage to seep past a seal that's good enough for regular oil. If a seal is weeping synthetic gear oil, chances are it's on its way out anyways.
Check for lateral play in the output yokes from the transfer case. Movement intdicates bearings that are in need of replacement. If the movement is greater than 1/8" chances are the gears themselves have been damaged once that occurrs, the transfer case gets EXTREMELY expensive to re-build.
If a 40 Series has been lifted more than 2", check to make sure that the notch in the skid plate the front drive shaft passes through has been enlarged. Otherwise, the rearmost yoke on the front driveshaft can bang on the plate causing the bearings in the transfer case to fail.
With the truck parked on a level surface, take out the transmission fill plug. If gear oil pours out of the plug, the seal between the transfer case and transmission is probably shot. This is a cheap part, but replacing it pretty much requires pulling the transfer and transmission. I also believe that when this seal goes, it's not a bad idea to rebuild the transfer anyways. It is usually the first internal problem that develops, and rebuilding the case when it goes ensures that the bearings do not get to far out of tollerance and allow the gears/shafts/etc. to become misaligned and wear excessively.
A leak from the rear output flange of the transfer usually results in destroyed parking brake shoes in pre-1981 transfer cases.
When test-driving the truck, feel how smoothly the truck shifts. It is normal for four and five speed transmissions to be a little balky when they're cold. The H55F 5 speed tends to be worse in this respect. If the transmission in a 40/55 Series is difficult to shift when warm, chances are that the synchros are shot.
60 Series and up H4x/H55F transmissions use an aluminum shift tower instead of the earlier cast-iron ones. There is a pin in either side of the shift tower that provides the pivot point for fore-aft movement of the shifter. Over time, they can begin to rock in the softer aluminum of the tower. If left long enough, they will totally work their way loose and even fall into the transmission. When this happens, it will become almost impossible to shift into 1st gear, even with the vehicle at a standstill.
H55F transmissions have another potential source of difficult shifting. There is a small nylon cup that fits on the end of the shifter where it engages the shift rails. Over time, the cup can wear which increases shifter slop and can make shifting more difficult. The cup is very inexpensive and relatively easy to replace.
The 3rd/4th/5th synchros are relatively inexpensive and easy to replace as part of a transmission rebuild, however the 1st/2nd synchro is more expensive than all the other synchros combined.
Transmissions/transfer cases popping out of gear are either the shifter binding against the boot, bad motor/tranny mounts or a desparate cry for a rebuild. If it's the latter and it's left for any length of time, severe gear/shift collar damage will result and rebuild costs will increase exponentially.
The 5th gear hub of H55F transmissions are more prone to this problem than the 4spd transmissions. Wear of the hub can start as a result of high loads on the transmission while in 5th gear (lugging up a hill, for example) or as a result of "rowing" back and forth between 5th and 4th while climging a hill to try to maintain speed. Wear of the needle bearings that the 5th counter-gear rides on can also allow the gear to rock slightly which wears its hub. The first sign of trouble is the transmission popping out of 5th gear when not under load.
As the bearings in the manual transmissions wear, a hissing sound will begin to resonate through the shifter. It is easier to hear on H41/42s because of the metal to metal connection between the shifter and rails as opposed to the H55F, with the nylon cup. The small diameter solid shifters also transmit the noise better than the rubber isolated shifters that increase in diameter just before they go through the shifter boot. The hissing will change tone slightly as the transmission goes from driving to coasting.