231 wont engage while driving.

Well i just finished my 8.8 swap, and while i did that i also changed my transfer case because the old one was fubared, and i installed an sye and front driveshaft. Anyways my problem now is that when i try to shift into 4hi while im driving at like 15mph it will just grind and not engage. If i stop and shift it then it works fine. This only happens while driving. Funny thing is, this is why i changed my other transfer case, well partially, it also used to make a horrible noise when in 4wd.

My jeep is a 88 with the vacuum disconnect so my np231 has those 4 vacuum lines on it, would this have anything to do with it? O and my old transfer case was a np231 amc, and i put in a np231J. Sorry to be so confusing but its winter here in Canada and it reallys sux to have to come to a complete stop to engage 4wd. Thanks.

What year is the transfercase you just put in. If it came out of a Jeep without the vacume disconect it won't have syncro's for shifting on the fly. This is because the front driveshaft is allready spinning at the same speed as the rear driveshaft do to the fact that both front axle shafts are always spinning. You can still shift it on the fly in the newer Jeeps but if you have a vacume disconnect your front driveshaft will not be spinning and it will grind...not good! I did away with the vacume disconnect and got a one piece axleshaft.

The transfer case came out of a 93 xj. For the first year that i drove my jeep it would shift on the fly no problem at all. the slip yoke that i put on has the hole for the vacuum lines to hook up also.

anyone?