99 express 1500 Rear End ?????

I'm having a problem with either the trans starting to slip in OD or maybe the rear end. When going 45-50 in OD, the van wants to shudder a little. I drop it back to 3rd and it goes away. Once running about 60-65 up a hill the whole van could feel the shudder. I have 120K and have changed the R.End fluid and trans filter & fluid around 88K on both. Where do you think I need to start looking.

Rear ends do not slip.
Start with engine running right.
When was last tune up?
Weak cylinder will give a jerking in OD or OD lockup.
Any flashing check engine lights when it does it?
Get it on a scanner and check for missfires and codes.
A transmission capable scanner will read out transmission and or transmission codes.
Has transmission ever been serviced?
Post back any codes.
Engine or transmission.

Just replaced the engine with a 96 sierra 5.7 from the junk yard that "had" about 40K. I've put about 1K on so far. The motor had set in my garage for a couple years so it may just need the cob webs cleaned out. Serviced the trans and the R end at about 88K.

Just replaced the engine with a 96 sierra 5.7 from the junk yard that "had" about 40K. I've put about 1K on so far. The motor had set in my garage for a couple years so it may just need the cob webs cleaned out. Serviced the trans and the R end at about 88K.

After 1K all cob webs should be burnt out.
Get it on a good scanner and check it out.
Let us know what you find.